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ACAD.lsp

Short answer

A LISP file AutoCAD looks for on every session start and loads automatically — the standard place to autoload routines.

ACAD.lsp is one of two startup LISP files AutoCAD scans for on launch (the other is acaddoc.lsp). When AutoCAD starts a new session, it searches the support file path for ACAD.lsp and executes any code it contains. ACAD.lsp runs once per session, before any drawing is open, which makes it the right place for session-level setup. For per-drawing setup (running every time a DWG is opened), use acaddoc.lsp instead.

Related guides

How to Auto-Load a LISP Routine When AutoCAD Starts

Four ways to make a LISP routine load automatically — Startup Suite, ACAD.lsp, acaddoc.lsp, and Trusted Locations — and which to pick.

Related terms

acaddoc.lsp

A LISP file AutoCAD loads every time a drawing is opened, used for per-document setup and (defun S::STARTUP).

AutoLISP

Autodesk's dialect of LISP, built into AutoCAD since 1986, used to write custom commands and automate drawings.

MNL file

A LISP file with the same base name as a CUIX/CUI menu, loaded automatically when that menu loads.