Real Dictionary

slump
verb: If something such as the value of something slumps, it falls suddenly and by a large amount.

slump in American English
a sudden or sharp fall
to fall, sink, or collapse, esp. suddenly or heavily
intransitive verb: to drop or fall heavily; collapse
noun: an act or instance of slumping

slump in British English
to sink or fall heavily and suddenly
noun
noun: a sudden or marked decline or failure, as in progress or achievement; collapse