/jen'sim/ [MacLISP for "generated symbol"] 1. To invent a new name for something temporary, in such a way that the name is almost certainly not in conflict with one already in use.
2. The resulting name. The canonical form of a gensym is "Gnnnn" where nnnn represents a number; any LISP hacker would recognise G0093 (for example) as a gensym.
3. A freshly generated data structure with a gensymmed name. Gensymmed names are useful for storing or uniquely identifying crufties (see cruft).