A setting or arrangement of a well-known tune (akin to its use in the jazz world).
The great standards in the 15th century were:

J’ay pris amours
De tous biens playne
Fors seulement
L’homme armé

In the 16th century they included:

Susanne un jour
Ancor che col partire
Frais (Frisque) et gaillard.