A defense alert was a type of generalized alert used by Starfleet in the 23rd century to indicate danger from a possible imminent threat.

In 2268, when the USS Enterprise was summoned to Deep Space Station K-7 on a priority 1 distress call, Captain James T. Kirk was aghast that Undersecretary for Agriculture Nilz Baris would place an entire quadrant on defense alert when there was no apparent emergency. (TOS: "The Trouble with Tribbles")