COM

Shorthand for "Communications," COM is the name for radio communications systems of UNSC units. COM allows for 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 MJOLNIR armor as well as the armor of Marines and ODSTs, as well as Pelicans, Longswords, and larger Star Ships.

Note that the Battlenet is the Covenant version of COM.

COM frequencies
FLEETCOM 7: Public frequency in use around the planet Reach. Broadcasts on FLEETCOM 7 reach any UNSC military ship in range of the sender.

SQUADCOM: Frequency reserved for use among small units in a specific area. Squadcom has a very small range. It is usually intended to be broadcast by small operations. Usually, communications that take place in Halo: Combat Evolved occur over a SQUADCOM.

TACCOM: Short for Tactical Communication. Frequency 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. Frequency 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, SPARTAN-044 used the E-Band to transmit a signal of Oly Oly Oxen Free. Used as an emergency COM Channel.

Other Stuff
In Halo 3 The Security Helmet Armor Permutation apparently has a COM radio on your back near your right shoulder, it is unusable.