Object

From GMod Wiki

Game Menu

In general game play, an object is anything you put in the game; a prop, an NPC, or any other "thing" that is visible on the screen that is not physically a part of the map. You create an object by using the Q key to open the game menu, then (if it is not already selected), you click on the props tab or the NPCs tab, then click on the selected object with the left mouse button. The object is then instantiated into the game at wherever your cursor crosshair is pointing.

Lua

In Garry's Mod Lua, Object is usually refered similar to "Class". For more details, see Class or Object in Wikipedia.