(GC) The process by which no-longer-used storage is reclaimed during the evaluation of an expression in a cell and pointer based memory system. In a system like Lisp which uses dynamic storage allocation and where expressions are represented as graphs containing pointers to cells or similar data structures it is necessary to reclaim space used by subexpressions but which is no longer pointed to by anything.
The three main methods are mark-sweep garbage collection, reference counting and copying garbage collection.
See also the AI koan about garbage collection.