Level editor
The tool where the game world actually gets built — walls, enemies, lights and triggers placed by hand by someone who doesn't need to write code.
id Software's DoomEd was written in 1993 on NeXTSTEP workstations, a long way from the DOS boxes the game would run on. Levels were drawn as 2D floor plans with floor and ceiling heights. The Quake era moved to brushes, convex blocks carved together with CSG, and that model lived on in Radiant, UnrealEd and Valve's Hammer. For 2D games, Tiled became the engine-agnostic default.
Modern engines are more like editors with a game inside: Unity and Godot let you edit the scene while it runs. Shipping the editor to players created the modding scene — which in turn produced Counter-Strike, DotA and half the industry's hiring pool. In 2015, Super Mario Maker made the editor itself the game.