once-only (name*) form*
Arguments and Values:
Meant to be used in macro code, once-only guards against multiple evaluation of its arguments in macroexpansion code. Any concise description would be far too vague to grasp, but this thread on comp.lang.lisp does a decent job of explaining what once-only does.
The description here is frustratingly non-descriptive, and I apologize for that. If you understand once-only and can give a better explanation, I would be very grateful—not to mention completely awed.