require

(require module-name [pathname])

Loads the file for a module unless the module was already registered by provide, then returns the module name as a symbol. module-name is a designator: a keyword (:util), a symbol ('util), or a string ("util"). Without an explicit pathname the file <name>.lisp is resolved relative to the requiring file, exactly like load; an explicit second argument overrides that mapping. The required file is expected to (provide <name>) itself, conventionally as its first form — that is what marks the module, so a second require of the same name is consumed without loading. This makes require the tool for the diamond dependency (a.lisp and b.lisp both requiring utils.lisp loads it once), where load would evaluate the file twice.

On the interpreter, require is an ordinary runtime function. On the compile path (JVM/WASM), a literal, top-level (require ...) is expanded at compile time: the module file is spliced into the program like the compile-time load include, so the compilers see its definitions natively. Unlike load, a require nested inside another form or with a computed argument is a compile error — it cannot be deferred to the compiled runtime reader. The Common Lisp *modules* variable is not available.

;; util.lisp
(provide :util)
(defun u-sq (x) (* x x))

;; main.lisp
(require :util)
(require :util)   ; already provided: no second load
(print (u-sq 7))  ; prints 49

The first require loads util.lisp, whose provide marks the module; the second is a no-op returning util.