The pyrrhic (the word is both the noun and the adjective) is a metrical foot of two unaccented syllables. The meter is common in classical Greek poetry, but most modern scholars do not use the term. Rather than identify the pyrrhic as a separate meter, they prefer to attach the unaccented syllables to adjacent feet.
In this line from Andrew Marvell's "The Garden" there are two pyrrhic feet that appear here in bold face: "To a | green thought | in a | green shade." Another example is this line from Lord Byron's Don Juan: "My way | is to | begin | with the | beginning."