A bacchius (/bəˈkəs/) is a metrical foot used in poetry.

Metrical feet and accents
˘ ˘pyrrhic, dibrach
˘ ¯iamb
¯ ˘trochee, choree
¯ ¯spondee
˘ ˘ ˘tribrach
¯ ˘ ˘dactyl
˘ ¯ ˘amphibrach
˘ ˘ ¯anapaest, antidactylus
˘ ¯ ¯bacchius
¯ ¯ ˘antibacchius
¯ ˘ ¯cretic, amphimacer
¯ ¯ ¯molossus

In accentual-syllabic verse we could describe a bacchius as a foot that goes like this:

da DUM DUM

Example:

When day breaks

the fish bite

at small flies.

The Christmas carol 'No Small Wonder' by Paul Edwards is a fair example of usage.