The term 'bottom' refers to a computation which never completes successfully. That includes a computation that fails due to some kind of error, and a computation that just goes into an infinite loop (without returning any data).
The mathematical symbol for bottom is '⊥'. That's Unicode character 22A5 hex = 8869 decimal. Also available in HTML as '⊥' and in LaTeX as '\perp' (within math mode). In plain ASCII, it's often written as the extremely ugly character sequence '