Shorthand for "Communications", COM is a term used of networks and radio communications systems of varying kind.
In civilian context, COM refers to communications channels, such as Public COM which appears to be similar to modern-day television or internet. It is known to house civilian network channels such as all-news channels. Public COM channels are also known to broadcast cartoons. COM pads, also known as "chatters", are also linked to public COM networks, and each has its own COM address. An "all-COM address" is an official broadcast sent to all communications devices on a given planet. Colonial Governors or other high-ranking officials have the authority to broadcast such transmission.
The term is often used to refer to the name for radio communications systems of UNSC units. COM allows instantaneous contact between people, ships and bases. There are any number of COM frequencies for any number of tasks. COM systems are standard equipment in all armor systems, as well as vehicles and ships.
SQUADCOM: Frequencies reserved for use among small units in a specific area. Squadcom has a very small range, it is usually intended to be broadcast over short distances. Usually, radio-communications that take place in Halo: Combat Evolved occur over a SQUADCOM.
TACCOM: Short for Tactical Communication. Frequencies reserved for sharing of information between small units on the battlefield. While audio communication is not possible on TACCOM, one team member can upload images, NAV points, and sensor information into the HUD of other members of their team. Similar to SQUADCOM.
SATCOM: Short for Satellite Communication. Frequencies reserved for communications with orbiting satellites, sometimes to relay information to orbiting ships, sometimes for communicating with people or units in another section of the planet. Note that COMSat stations must be in orbit in order for SATCOM to be usable.
E-BAND: Emergency broadcast channel. Little-used public frequency. After the fall of Reach, Anton-044 used the E-Band to transmit a signal of Oly Oly Oxen Free. It was also used by the UNSC forces on Onyx to notify Lord Hood's Reinforcements of their position. Used as an emergency COM Channel.
HIGHCOM: Short for "High Command", this band is used by and to contact the ranking officers of space ships.