Entity

Every object that exists in the game world is an entity. Entities may be players, mobs, NPCs, walls, bullets, backgrounds, triggers, etc. Anything other than UI that is rendered (or not rendered) in the engine will be an entity.

Tilesheet

A tilesheet is a single image composed of multiple images or tiles. Another name for a tilesheet is also spritesheet.
The use of tilesheets is recommended for performance in firmament engine. 
Animations also require that all frames of an animation be in the same tilesheet.