defmacro

(defmacro name doc? attr? [params] body...) / (defmacro name doc? attr? ([params] body...)+)

Defines a compile-time macro: each call site expands while lowering, before any backend runs, so every backend executes expanded code. Parameters bind the call's argument forms unevaluated (& the rest as a list, destructuring like defn parameters); several arities dispatch on the argument count, like defn. A docstring and an attr map are skipped. &form and &env are refused: a body runs with its arguments only, never with a compilation environment.

The definition answers nil and registers a runtime table entry of the same expander, so macroexpand-1 expands the same function at run time. A macro body sees the core builtins and the clojure.lisp library, not the program's own definitions; a call above its definition is an error, and a macro has no function value.

A defmacro of a core name -- a function like inc or a form like with-out-str -- shadows it from its definition on: a call site above the definition keeps the core meaning, like the oracle's form-by-form compile, and clojure.core/name names the core var whatever the program defines. A special form (if, do, let*, new, ...) or a head the reader spells (deref, with-meta, syntax-quote, ns, in-ns) cannot name a macro.