hair·spring 
(hârsprĭng′)
(hârsprĭng′)n.
A fine coiled spring that regulates the movement of the balance wheel in a watch or clock.
- tetraethyl lead
- tetraethyllead
- tetraethyl pyrophosphate
- tetragon
- tetragonal
- tetragonally
- Tetragrammaton
- tetrahedral
- tetrahedrally
- tetrahedrite
- tetrahedron
- tetrahydrocannabinol
- tetrahydrofuran
- tetrahydroxy
- tetralogy
- tetralogy of Fallot
- tetramer
- tetrameric
- tetramerous
- tetramery
- tetrameter
- tetraplegia
- tetraplegic
- tetraploid
- tetraploidy