In ichthyology, the length of a fish as measured from the snout to the base of the caudal fin. More precisely, it is the shortest distance from the front of the upper lip to the posterior end of the vertebral column. In most bony fishes (class Osteichthyes) the vertebral column ends with a broadened hypural plate that supports the rays of the caudal fin. This point is detected in small, dead specimens by bending the caudal fin to one side; in live fish, it's estimated to be at the end of the caudal peduncle.
Standard length is important in taxonomy because it removes deviation due to the state of the caudal fin, a problem that shows up when using total length. It's an important component of morphometrics, where it's often used in ratios with other body measurements to capture proportion.