A call sign is a nickname adopted by a military pilot for use in name tags, planes and radio conversations. Though strictly unofficial, these names are adopted almost like a second name for the person they are attached to and thus, they are usually given by other pilots or coworkers. Each name varies and the reasons they are given vary as well. Several characters in Halo are assigned call signs.
A call sign is a nickname given to Starfleet fighter pilots. The call sign is frequently used in radio conversations, and is typically noted on the sides of a pilot's assigned fighter.
It is considered bad form to give oneself a call sign. Typically, call signs are given by one's co-workers.
Roslyn earned the call sign "Firecracker" from Johnathan Bucyrus for her fiery temper, and for her skills in basic fighter training. Roslyn subsequently gave Johnathan the call sign "Bulldog." (USS Baldwin: "Slow Burn", "Wild Roses")
Melora Lexis obtained the call sign "Hat Trick" by destroying three Cardassian fighters with only a single missile. She had previolsly received the call sign "Posh" for letting slip that she was looking forward to a perm and a manicure following jungle survival training. (Starship Independence: "A Dish Best Served Cold")
Call signs were frequency names for a members of starfighter squadrons, used to save communication time and confusion by assigning each pilot a name or number. It is also a name for a squad on a communications net. Infantry squads also used call signs.