Command and control
Overview: 

US perspective: The US Department of Defense Dictionary of Military and Associated Terms defines command and control as: "The exercise of authority and direction by a properly designated commander over assigned and attached forces in the accomplishment of the mission. Also called C2. Source: JP 1".
The edition of the Dictionary "As Amended Through April 2010" elaborates, "Command and control functions are performed through an arrangement of personnel, equipment, communications, facilities, and procedures employed by a commander in planning, directing, coordinating, and controlling forces and operations in the accomplishment of the mission." However, this sentence is missing from the "command and control" entry for the edition "As Amended Through 15 August 2014."
Commanding officers are assisted in executing these tasks by specialized staff officers and enlisted personnel. These military staff are a group of officers and enlisted personnel that provides a bi-directional flow of information between a commanding officer and subordinate military units.
The purpose of a military staff is mainly that of providing accurate, timely information which by category represents information on which command decisions are based. The key application is that of decisions that effectively manage unit resources. While information flow toward the commander is a priority, information that is useful or contingent in nature is communicated to lower staffs and units.

Computer security industry: This term is also in common use within the computer security industry and in the context of cyberwarfare. Here the term refers to the influence an attacker has over a compromised computer system that they control. For example, a valid usage of the term is to say that attackers use "command and control infrastructure" to issue "command and control instructions" to their victims. Advanced analysis of command and control methodologies can be used to identify attackers, associate attacks, and disrupt ongoing malicious activity.

Derivative terms: There is a plethora of derivative terms that emphasize various aspects, uses, and sub-domains of C2. These terms are accompanied by numerous associated abbreviations. For example, in addition to C2, command and control is often abbreviated as C2 and sometimes as C&C

"Command and control" have been coupled with:

Collaboration
Communication / communications
Computers / computing
Electronic warfare
Interoperability
Reconnaissance
Surveillance
Target acquisition
and others.

Some of the more common variations include:

AC2 - Aviation command & control
C2I – Command, control & intelligence
C2I – command, control & information (a less common usage)
R2C2I - rapid advanced manufacturing, command, control & intelligence [developed by SICDRONE]
C2IS – command and control information systems
C2ISR – C2I plus surveillance and reconnaissance
C2ISTAR – C2 plus ISTAR (intelligence, surveillance, target acquisition, and reconnaissance)
C3 – command, control & communication (human activity focus)
C3 – command, control & communications (technology focus)
C3 – consultation, command, and control [NATO]
C3I – 4 possibilities; the most common is command, control, communications and intelligence
C3ISTAR – C3 plus ISTAR
C3ISREW – C2ISR plus communications plus electronic warfare (technology focus)
C3MS - cyber command and control mission system
C3/SA - C3 plus situational awareness
C4, C4I, C4ISR, C4ISTAR, C4ISREW, C4ISTAREW – plus computers (technology focus) or computing (human activity focus)
C4I2 – command, control, communications, computers, intelligence, and interoperability
C5I – command, control, communications, computers, collaboration and intelligence
C5I – command, control, communications, computers, cyber and intelligence (US Army)
C6ISR – command, control, communications, computers, cyber-defense and combat systems and intelligence, surveillance, and reconnaissance
MDC2 - multi-domain command and control
NC2 − nuclear command and control
NC3 − nuclear command and control and communications
and others.

Command: The exercise of authority based upon certain knowledge to attain an objective.
Control: The process of verifying and correcting activity such that the objective or goal of command is accomplished.
Communication: Ability to exercise the necessary liaison to exercise effective command between tactical or strategic units to command.
Computers: The computer systems and compatibility of computer systems. Also includes data processing.
Intelligence: Includes collection as well as analysis and distribution of information.

Command and control centers: A command and control center is typically a secure room or building in a government, military or prison facility that operates as the agency's dispatch center, surveillance monitoring center, coordination office and alarm monitoring center all in one. Command and control centers are operated by a government or municipal agency.
Various branches of the US military such as the US Coast Guard and Navy have command and control centers. They are also common in many large correctional facilities.
A command and control center that is used by a military unit in a deployed location is usually called a "command post". A warship has a combat information center for tactical control of the ship's resources, but commanding a fleet or joint operation requires additional space for commanders and staff plus C4I facilities provided on a flagship (e.g., aircraft carriers), sometimes a command ship or upgraded logistics ship such as USS Coronado.

Command and control warfare: Command and control warfare encompasses all the military tactics that use communications technology. It can be abbreviated as C2W. An older name for these tactics is "signals warfare", derived from the name given to communications by the military. Newer names include information operations and information warfare.
The following techniques are combined:

Cyber operations
with the physical destruction of enemy communications facilities. The objective is to deny information to the enemy and so disrupt its command and control capabilities. At the same time precautions are taken to protect friendly command and control capabilities against retaliation.
In addition to targeting the enemy's command and control, information warfare can be directed to the enemy's politicians and other civilian communications.

Electronic warfare (EW)
Military deception
Operations security (OPSEC)
Psychological operations (PSYOP)
Psychological warfare

See also: US and other NATO specific:

Other

Military Institute of Telecommunications and Information Technologies

References: 

Citations: 

Sources: 

External links: 
Command and control definitions and procedures, UK College of Policing
International Command and Control Institute
Understanding Command and Control by D. S. Alberts and R. E. Hayes (2006)