The fire at the centre of a dwelling, and the shared warmth that gathers people to it. What is common to everyone in the house.
Old English heorth (hearth, fireplace), from Proto-Germanic *herthō, from a Proto-Indo-European root *ker- (heat, fire). Cognate with German Herd and Dutch haard. For most of the word's life it named the literal floor of the fire — and, by inevitable extension, the household gathered at it: the fire you keep going is the reason people stay in the room.
The regulation realm — breath, timers, the sensory doors. The oldest technology for staying alive in the cold is a fire someone tends, and the realm is built on the same premise: regulation is not a cure applied once but a warmth kept up.