(Unicode character U+22A5: "up tack")
"Nothing". The abstract subtype of all types; the strongest and most specific type.
Bottom cannot have any instances because they would be logically inconsistent.
Bottom serves as the "return type" for functions which cannot return (because of program exit, infinite looping, unhandled exceptions, or calling a continuation).
References: An in-depth exploration of ⊥
See also: Top