Infantry
Etymology and terminology: In English, use of the term infantry began about the 1570s, describing soldiers who march and fight on foot. The word derives from Middle French infanterie, from older Italian (also Spanish) infanteria (foot soldiers too inexperienced for cavalry), from Latin īnfāns (without speech, newborn, foolish), from which English also gets infant. The individual-soldier term infantryman was not coined until 1837. In modern usage, foot soldiers of any era are now considered infantry and infantrymen.
From the mid-18th century until 1881, the British Army named its infantry as numbered regiments "of Foot" to distinguish them from cavalry and dragoon regiments (see List of Regiments of Foot).
Infantry equipped with special weapons were often named after that weapon, such as grenadiers for their grenades, or fusiliers for their fusils. These names can persist long after the weapon speciality; examples of infantry units that retained such names are the Royal Irish Fusiliers and the Grenadier Guards.
Dragoons were created as mounted infantry, with horses for travel between battles; they were still considered infantry since they dismounted before combat. However, if light cavalry was lacking in an army, any available dragoons might be assigned their duties; this practice increased over time, and dragoons eventually received all the weapons and training as both infantry and cavalry, and could be classified as both. Conversely, starting about the mid-19th century, regular cavalry have been forced to spend more of their time dismounted in combat due to the ever-increasing effectiveness of enemy infantry firearms. Thus most cavalry transitioned to mounted infantry. As with grenadiers, the dragoon and cavalry designations can be retained long after their horses, such as in the Royal Dragoon Guards, Royal Lancers, and King's Royal Hussars.

Similarly, motorised infantry have trucks and other unarmed vehicles for non-combat movement, but are still infantry since they leave their vehicles for any combat. Most modern infantry have vehicle transport, to the point where infantry being motorised is generally assumed, and the few exceptions might be identified as modern light infantry. Mechanised infantry go beyond motorised, having transport vehicles with combat abilities, armoured personnel carriers (APCs), providing at least some options for combat without leaving their vehicles. In modern infantry, some APCs have evolved to be infantry fighting vehicles (IFVs), which are transport vehicles with more substantial combat abilities, approaching those of light tanks. Some well-equipped mechanised infantry can be designated as armoured infantry.  Given that infantry forces typically also have some tanks, and given that most armoured forces have more mechanised infantry units than tank units in their organisation, the distinction between mechanised infantry and armour forces has blurred.

History: The first military forces in history were infantry. In antiquity, infantry were armed with early melee weapons such as a spear, axe, or sword, or an early ranged weapon like a javelin, sling, or bow, with a few infantry men being expected to use both a melee and a ranged weapon. With the development of gunpowder, infantry began converting to primarily firearms. By the time of Napoleonic warfare, infantry, cavalry and artillery formed a basic triad of ground forces, though infantry usually remained the most numerous. With armoured warfare, armoured fighting vehicles have replaced the horses of cavalry, and airpower has added a new dimension to ground combat, but infantry remains pivotal to all modern combined arms operations.
The first warriors, adopting hunting weapons or improvised melee weapons, before the existence of any organised military, likely started essentially as loose groups without any organisation or formation. But this changed sometime before recorded history; the first ancient empires (2500–1500 BC) are shown to have some soldiers with standardised military equipment, and the training and discipline required for battlefield formations and manoeuvres: regular infantry. Though the main force of the army, these forces were usually kept small due to their cost of training and upkeep, and might be supplemented by local short-term mass-conscript forces using the older irregular infantry weapons and tactics; this remained a common practice almost up to modern times.

Before the adoption of the chariot to create the first mobile fighting forces c. 2000 BC, all armies were pure infantry. Even after, with a few exceptions like the Mongol Empire, infantry has been the largest component of most armies in history.
In the Western world, from Classical Antiquity through the Middle Ages (c. 8th century BC to 15th century AD), infantry are categorised as either heavy infantry or light infantry. Heavy infantry, such as Greek hoplites, Macedonian phalangites, and Roman legionaries, specialised in dense, solid formations driving into the main enemy lines, using weight of numbers to achieve a decisive victory, and were usually equipped with heavier weapons and armour to fit their role. Light infantry, such as Greek peltasts, Balearic slingers, and Roman velites, using open formations and greater manoeuvrability, took on most other combat roles: scouting, screening the army on the march, skirmishing to delay, disrupt, or weaken the enemy to prepare for the main forces' battlefield attack, protecting them from flanking manoeuvers, and then afterwards either pursuing the fleeing enemy or covering their army's retreat.
After the fall of Rome, the quality of heavy infantry declined, and warfare was dominated by heavy cavalry, such as knights, forming small elite units for decisive shock combat, supported by peasant infantry militias and assorted light infantry from the lower classes. Towards the end of Middle Ages, this began to change, where more professional and better trained light infantry could be effective against knights, such as the English longbowmen in the Hundred Years' War. By the start of the Renaissance, the infantry began to return to a larger role, with Swiss pikemen and German Landsknechts filling the role of heavy infantry again, using dense formations of pikes to drive off any cavalry.
Dense formations are vulnerable to ranged weapons. Technological developments allowed the raising of large numbers of light infantry units armed with ranged weapons, without the years of training expected for traditional high-skilled archers and slingers. This started slowly, first with crossbowmen, then hand cannoneers and arquebusiers, each with increasing effectiveness, marking the beginning of early modern warfare, when firearms rendered the use of heavy infantry obsolete. The introduction of musketeers using bayonets in the mid 17th century began replacement of the pike with the infantry square replacing the pike square.

To maximise their firepower, musketeer infantry were trained to fight in wide lines facing the enemy, creating line infantry. These fulfilled the central battlefield role of earlier heavy infantry, using ranged weapons instead of melee weapons. To support these lines, smaller infantry formations using dispersed skirmish lines were created, called light infantry, fulfilling the same multiple roles as earlier light infantry. Their arms were no lighter than line infantry; they were distinguished by their skirmish formation and flexible tactics.
The modern rifleman infantry became the primary force for taking and holding ground on battlefields as an element of combined arms. As firepower continued to increase, use of infantry lines diminished, until all infantry became light infantry in practice. Modern classifications of infantry have since expanded to reflect modern equipment and tactics, such as motorised infantry, mechanised or armoured infantry, mountain infantry, marine infantry, and airborne infantry.

Equipment: Beyond main arms and armour, an infantryman's "military kit" generally includes combat boots, battledress or combat uniform, camping gear, heavy weather gear, survival gear, secondary weapons and ammunition, weapon service and repair kits, health and hygiene items, mess kit, rations, filled water canteen, and all other consumables each infantryman needs for the expected duration of time operating away from their unit's base, plus any special mission-specific equipment. One of the most valuable pieces of gear is the entrenching tool—basically a folding spade—which can be employed not only to dig important defences, but also in a variety of other daily tasks, and even sometimes as a weapon. Infantry typically have shared equipment on top of this, like tents or heavy weapons, where the carrying burden is spread across several infantrymen. In all, this can reach 25–45 kg (60–100 lb) for each soldier on the march. Such heavy infantry burdens have changed little over centuries of warfare; in the late Roman Republic, legionaries were nicknamed "Marius' mules" as their main activity seemed to be carrying the weight of their legion around on their backs, a practice that predates the eponymous Gaius Marius.

When combat is expected, infantry typically switch to "packing light", meaning reducing their equipment to weapons, ammunition, and other basic essentials, and leaving other items deemed unnecessary with their transport or baggage train, at camp or rally point, in temporary hidden caches, or even (in emergencies) simply discarding the items. Additional specialised equipment may be required, depending on the mission or to the particular terrain or environment, including satchel charges, demolition tools, mines, or barbed wire, carried by the infantry or attached specialists.
Historically, infantry have suffered high casualty rates from disease, exposure, exhaustion and privation — often in excess of the casualties suffered from enemy attacks. Better infantry equipment to support their health, energy, and protect from environmental factors greatly reduces these rates of loss, and increase their level of effective action.