A stout timberpost or strut with attendant wedge used to exert pressure on a large object when glueing.
Used in boatbuilding and often the Thom is braced against the floor of the shop or against the slipway cradle. "Let's set up a Thom and get some pressure to bear when we glue in the scarfed piece", "We should Thom the skeg so she bottoms out in the rebate".