A virgule is the slash mark (/). In poetry it is often used to show a line break in quoted verse or to mark divisions in a scanned line, especially when a poem is being discussed or copied into prose.
Because it can stand for either a line break or a metrical division, the virgule should be read in context rather than treated as a fixed poetic rule.
Example
The wind moved through the reeds / and the lake went still,
while one late star / held the dark in place.