A Code One Alert (sometimes referred to as a Priority One Alert), is an emergency distress call used in the 23rd century. More than an emergency, it signals near or total disaster, invasion, or the outbreak of warfare.
Starfleet used Code One to signal the commencement of open warfare with the Klingon Empire in the mid 23rd century. (TOS: "Errand of Mercy")
Code One was again invoked when a galaxy-wide energy phenominon was suspected to be a prelude to an invasion. (TOS: "The Alternative Factor")
In 2268, Deep Space Station K-7 summoned the Enterprise by issuing a Priority One alert, under the order of Federation Undersecretary Nilz Baris. (TOS: "The Trouble with Tribbles")