A pyrrhic is a metrical foot used in formal poetry

Poetry

Poetry is a form of literary art in which language is used for its aesthetic and evocative qualities in addition to, or in lieu of, its apparent meaning...

. It consists of two unaccented, short syllable

Syllable weight

In linguistics, syllable weight is the concept that syllables pattern together according to the number and/or duration of segments in the rime. In classical poetry, both Greek and Latin, distinctions of syllable weight were fundamental to the meter of the line....

s. It is also known as a dibrach.

Tennyson used pyrrhics and spondee

Spondee

In poetry, a spondee is a metrical foot consisting of two long syllables, as determined by syllable weight in classical meters, or two stressed syllables, as determined by stress in modern meters...

s quite frequently, for example, in In Memoriam

In Memoriam A.H.H.

In Memoriam A.H.H. is a poem by the English poet Alfred, Lord Tennyson, completed in 1849. It is a requiem for the poet's Cambridge friend Arthur Henry Hallam, who died suddenly of a cerebral haemorrhage in Vienna in 1833...

: "When the blood creeps and the nerves prick." "When the" and "and the" in the second line may be considered as pyrrhics (also analyzable as ionic meter

Ionic meter

The ionic is a four-syllable metrical unit of light-light-heavy-heavy that occurs in ancient Greek and Latin poetry. Like the choriamb, in classical quantitative verse the ionic never appears in passages meant to be spoken rather than sung...

).

Pyrrhics alone are not used to construct an entire poem due to the monotonous effect. Poe observed that many experts rejected it from English metrics and concurred:

The pyrrhic is rightfully dismissed. Its existence in either ancient or modern rhythm is purely chimerical, and the insisting on so perplexing a nonentity as a foot of two short syllables, affords, perhaps, the best evidence of the gross irrationality and subservience to authority which characterise our Prosody.