(covers information from several alternate timelines)
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 weapon, the captain's bridge command chair was fitted with slots for shoulder harness seatbelts. (Star Trek: Enterprise fourth season)
As of 2164, twin-shoulder seatbelts were featured on the bridge and in engineering aboard the Franklin-type USS Franklin. (Star Trek Beyond)
The USS Enterprise of the alternate reality featured twin-shoulder belts for times of gravity failure, though they – unlike those aboard the Franklin – were self-extending. (Star Trek Into Darkness)
Constitution-class starships 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")