A seatbelt was a harness designed to hold and protect a passenger of a vehicle in case of an accident or a sudden stop.
On NX-class starships following the destruction of the Xindi superweapon, the captain's bridge command chair was fitted with slots for shoulder harness seatbelts. (Star Trek: Enterprise fourth season)
The USS Enterprise featured self-extending twin-shoulder belts for times of gravity failure. (Star Trek Into Darkness)
Constitution-class starship were equipped with seatbelts, which were employed in cases when there was a loss of gravity on the bridge. (TAS: "Once Upon a Planet")
Starfleet type 15 shuttlepods were equipped with seatbelts for all passengers. (TNG: "Power Play")