A greenhouse (also called a glasshouse) is a structure with walls and roof made mainly of transparent material, such as glass, in which plants requiring regulated climatic conditions are grown.

A more scientific definition is "a covered structure that protects the plants from extensive external climate conditions and diseases, creates optimal growth micro-environment, and offers a flexible solution for sustainable and efficient year-round cultivation."

A modern greenhouse operates as a system, therefore it is also referred to as controlled environment agriculture (CEA), controlled environment plant production system (CEPPS), or phytomation system.