What is the defining feature of a "pyrrhic"?

The defining feature of a "pyrrhic" is indeed two unstressed syllables. This metrical foot is characterized by its use of two Syls that lack a strong emphasis or stress, making it distinct in rhythm and structure compared to other metrical feet. In contrast to other options that include stressed syllables, which alter the rhythm and emphasis within poetry, the pyrrhic's unique combination focuses entirely on maintaining a light, unaccented flow. This can create specific effects in poetic meter, contributing to the overall sound and rhythm of a poem.