Biceps is a point in a metrical pattern that can be filled either with one long syllable (a longum) or two short syllables (brevia). It is found in the dactylic hexameter and the dactylic pentameter.
It is not to be confused with resolution, which is the replacement of a long with two shorts. Resolution is carefully limited within a line, whereas a biceps can freely be either long or two shorts.
References
Biceps (prosody) Wikipedia(Text) CC BY-SA