Plain English Meaning
A switch that automatically turns the cabin heater off if it gets too hot, so the heater cannot become a fire hazard.
Full Definition
A safety device in a combustion-type aircraft heating system that automatically shuts the heater off when internal temperatures exceed a safe limit. It protects against fire and damage caused by a malfunctioning heater, and once tripped, it typically requires manual reset on the ground after the cause has been investigated.
Context Anchor
Seen in aircraft cabin heating system descriptions, especially for fuel-burning heaters used to warm cabin air.