In-game level editor
The construction kit or similar works in-game rather than as separate tool, often adding a layer of control on top of the game rather than a separate editor, making level testing simpler and faster while maintaining familiar controls.
Alternate names: In-engine editor, In-game construction kit
The first video game about In-game level editor was released in 1981.
This is NOT for instances where the editor is accessible in-game but works like a separate tool.