One of the standard Western musical modes. The name had one meaning to the ancient Greeks, then was given a different meaning by medieval theorists who misunderstood the Greek treatises.
B C D E F G A B s t t s t t t
Note that this example illustrates the diatonic genus -- the various ancient Greek modes had variant interval structures, depending on the genus and shade.
In ecclesiastical and modern usage, dating from the incorrect application of Boethius's names to the system of church modes in the Alia Musica (c.870), the mixolydian mode is the diatonic octave-species from G to G, in ascending order thus ("t" = whole-tone, "s" = semitone):
G A B C D E F G t t s t t t s