Bathos is an abrupt and often awkward drop from serious, elevated language to something ordinary, sentimental, or absurd. In poetry, it usually appears as an anticlimax: a passage builds toward grandeur, then lands in a trivial or ridiculous detail. Writers may use bathos deliberately for comedy, but it often describes an unintended failure of style.
This expression comes from Pope's satire Peri Bathous, or the Art of Sinking (1727).
Example
The thunder rolled; the armies shook the plain-
then Martha called us in for soup and rain.