DCL
Short answer
Dialog Control Language — a small markup AutoCAD uses to define LISP dialog boxes.
DCL (Dialog Control Language) is AutoCAD's legacy file format for defining LISP dialog boxes. A .dcl file declares the layout — rows, columns, buttons, edit boxes — and LISP code uses load_dialog, new_dialog, action_tile, and start_dialog to wire it up. DCL is dated but still ubiquitous because it requires zero external dependencies and works in every AutoCAD vertical.