eval-when

(eval-when (situation...) body...)

Evaluates the body as a progn, treating every situation (:compile-toplevel, :load-toplevel, :execute, and the deprecated compile/load/eval spellings) as "evaluate now". The situation list is required but otherwise ignored, so code that runs under real Common Lisp situation rules also runs here.

At top level the body forms are spliced into the surrounding program on the compile path, so the common macro-exporting idiom -- a defmacro wrapped in (eval-when (:compile-toplevel :load-toplevel :execute) ...) -- defines the macro for the rest of the compilation unit, and nested defuns are collected like ordinary top-level definitions.