Loitering munition
History: 

First development and terminology: Initially, loitering munitions were not referred to as such but rather as "suicide UAVs" or "loitering missiles". Different sources point at different projects as originating the weapon category. The failed US AGM-136 Tacit Rainbow program or the 1980s initial Israeli Delilah variants are mentioned by some sources. The Iranian Ababil-1 was produced in the 1980s but its exact production date is unknown. The Israeli IAI Harpy was produced in the late 1980s.

Early projects did not use the "loitering munition" nomenclature, which emerged much later; they used terminology existing at the time. For instance the AGM-136 Tacit Rainbow was described in a 1988  article: the Tacit Rainbow unmanned jet aircraft being developed by Northrop to loiter on high and then swoop down on enemy radars could be called a UAV, a cruise missile, or even a standoff weapon. But it is most definitely not an RPV.

Initial role in suppression of enemy air defense: The response to the first generation of fixed installation surface-to-air missiles (SAMs) such as S-75 and S-125 was the development of the anti-radiation missiles (ARMs) such as AGM-45 Shrike and other means to attack fixed SAM installations, as well as developing SEAD doctrines. The Soviet counter-response was the use of mobile SAMs such as 2K12 Kub with intermittent use of radar. Thus, the SAM battery was only visible for a small period of time, during which it was also a significant threat to high-value Wild Weasel fighters. In Israel's 1982 Operation Mole Cricket 19 various means including UAVs and air-launched Samson decoys were used over suspected SAM areas to saturate enemy SAMs and to bait them to activate their radar systems, which were then attacked by ARMs.
In the 1980s, a number of programs, such as the IAI Harpy or the AGM-136 Tacit Rainbow, integrated anti-radiation sensors into a drone or missile air frames coupled with command and control and loitering capabilities. This allowed the attacking force to place relatively cheap munitions in place over suspected SAM sites, and to attack promptly the moment the SAM battery is visible. This integrated the use of a drone as a baiting decoy with the attack role into one small and relatively cheap platform in comparison to the alternative wild weasel jet fighter.

Evolution into additional roles: Starting in the 2000s, loitering weapons have been developed for additional roles beyond the initial SEAD role, ranging from relatively long-range strikes and fire support down to tactical, very short-range battlefield use. A documented use of loitering munitions was in the 2016 Nagorno-Karabakh conflict in which an IAI Harop was used against a bus being used as a troop transport for Armenian soldiers. The ZALA Lancet and several Shahed drones, including the HESA Shahed 136, have been used by Russia in the ongoing Russian invasion of Ukraine, while Ukraine has fielded loitering munitions such as the UJ-25 Skyline or the American-made AeroVironment Switchblade, which is deployed at the platoon level and fits in a backpack.
During conflicts in the 2010s and 2020s, conventional armies and non-state militants alike began modifying common commercial racing drones into an "FPV loitering munition" by the attachment of a small explosive, so-named because of the first-person view (FPV) they provide the operator. Explosive ordnance such as an IED, grenade, mortar round or an RPG warhead are fitted to an FPV drone then deployed to aerial bomb tactical targets. FPV drones also allow direct reconnaissance during the drone's strike mission.
After the Russian invasion of Ukraine began in 2022, both Russian and Ukrainian forces were producing thousands of FPV drones every month by October 2023, many of which were donated by volunteer groups. Escadrone Pegasus and the Vyriy Drone Molfar are two examples of the low-cost drones that rapidly evolved in 2022–23 during the war.  In 2022, the UK Government announced it was providing "hundreds of loitering munitions" to Ukraine. On 9 November 2023, Ukrainian soldiers claimed to have used a civilian-donated FPV drone to destroy a Russian Tor missile system on the Kupiansk front, showcasing the potential cost-effectiveness of fielding such munitions. A Tor missile system costs some $24 million dollars to build, which could buy 14,000 FPV drones.

Characteristics: Loitering munitions may be as simple as an unmanned aerial vehicle (UAV) with attached explosives that is sent on a potential kamikaze mission, and may even be constructed with commercially-available quadcopters with strapped-on explosives.
Purpose-built munitions are more elaborate in flight and control capabilities, warhead size and design, and onboard sensors for locating targets. Some loitering munitions use a human operator to locate targets whereas others, such as IAI Harop, can function autonomously searching and launching attacks without human intervention. Another example is UVision HERO solutions – the loitering systems are operated remotely, controlled in real time by a communications system and equipped with an electro-optical camera whose images are received by the command and control station.
Some loitering munitions may return and be recovered by the operator if they are unused in an attack and have enough fuel; in particular this is characteristic of UAVs with a secondary explosive capability. Other systems, such as the Delilah do not have a recovery option and are self-destructed in mission aborts.

Countermeasures: Russia uses ZALA Lancet drones in Ukraine. Since spring 2022 Ukrainian forces have been building cages around their artillery pieces using chain link fencing, wire mesh and even wooden logs as part of the construction. One analyst told Radio Liberty that such cages were "mainly intended to disrupt Russian Lancet munitions." A picture supposedly taken from January 2023 shows the rear half of a Lancet drone that failed to detonate due to such cages. Likewise Ukrainian forces have used inflatable decoys and wooden vehicles, such as HIMARS, to confuse and deceive Lancet drones.
Ukrainian soldiers report shooting down Russian drones with sniper rifles. Russian soldiers use electronic warfare to disable or misdirect Ukrainian drones and have reportedly used the Stupor anti-drone rifle, which uses an electromagnetic pulse that disrupts a drone's GPS navigation. A Royal United Services Institute study in 2022 found that Russian Electronic Warfare units, in March and April 2022, knocked out or shot down 90% of Ukrainian drones that they had at the start of the war in February 2022. The main success was in jamming GPS and radio links to the drones.
Both Ukraine and Russia rely on electronic warfare to defeat FPV drones. Such jammers are now used on Ukrainian trenches and vehicles. Russian forces have built jammers that can fit into a backpack. And now pocket-size jammers exist for soldiers. As of June 2023 Ukraine was losing 5-10,000 drones a month, or 160 per day, according to 
Ukrainian soldiers.
This has led to Russia creating wire guided FPV drones, similar to a wire-guided missile or even wire-guided torpedoes. One drone captured by Ukrainian forces had 10.813 km of fibre optic cable. Such guidance would make the link between operators and FPV drone immune to jamming. It would also allow for much faster updates from the drone. However these drones would lack the manoeuvrability that wireless drones enjoy. Ukraine has also responded by using autonomous drones tasking to ensure that a jammed drone can hit a target. In March 2024 footage put on social media showed a Ukrainian FPV drone being jammed just before it struck a target. Despite the loss of operator control it still managed to strike the target.
Russian tanks have been fitted with rooftop slat armor at the beginning of the Russian invasion of Ukraine which could provide protection against loitering munitions in some circumstances. Some Ukrainian tanks taking part in the 2023 Ukrainian counteroffensive were also spotted using roof screens.
On 21 March 2024, recent footage of the submarine Tula showed that it has been fitted with a slat armor to prevent drone strikes, the first ocean-going asset to carry such a modification.

Comparison to similar weapons: Loitering munitions fit in the niche between cruise missiles and unmanned combat aerial vehicles (UCAVs).
The following table compares similar size-class cruise missiles, loitering munitions, and UCAVS:

Whereas some cruise missiles, such as the Block IV Tomahawk, have the ability to loiter and have some sensory and remote control features, their primary mission is typically strike and not target acquisition. Cruise missiles, as their name implies, are optimized for long-range flight at constant speed both in terms of propulsion systems and wings or lifting body design. They are often unable to loiter at slow fuel-efficient speeds which significantly reduces potential loiter time even when the missile has some loiter capabilities.
Conversely almost any UAV could be piloted to crash onto a target and most could be fitted with an improvised explosive warhead. However the primary use of a UAV or UCAV would be for recoverable flight operations carrying reconnaissance equipment and/or munitions.