In Ancient Greek and Latin meter, brevis in longo ([ˈbrɛwɪs ɪn ˈlɔŋgoː]) is a short syllable at the end of a line that is counted as long.
The term means "a short [syllable] in place of a long [syllable]." Brevis in longo is possible in any classical meter that requires a long syllable at the end of a line, including dactylic hexameter and iambic trimeter.
Brevis in longo is quite distinct from the metrical element anceps, which is a syllable that can be either short or long. These two phenomena are often confused but there are differences between the two. For example, an anceps will be considered short or long in accordance with its natural length. A brevis in longo, on the other hand, will always be considered long, even though its natural quantity is short: the pause at the end of the line adds weight enough for even a short syllable to be counted as long.
An additional distinction is the following: some classical meters have an anceps syllable in certain positions in the line (at the beginning of each metron, for example, in iambic trimeter). The placement of the anceps is dictated by the type of meter. However, all classical meters have the possibility of a brevis in longo, proving that the brevis in longo is a different phenomenon.