hair·spring 
(hârsprĭng′)
(hârsprĭng′)n.
A fine coiled spring that regulates the movement of the balance wheel in a watch or clock.
- Artaxerxes II
- retrofittable
- retrofitter
- retroflection
- retroflex
- retroflexion
- retrogradation
- retrograde
- retrograde ejaculation
- retrogradely
- retrogress
- retrogression
- retrogressive
- retrogressively
- retrolental
- retronym
- retroocular
- retroperitoneal
- retropharyngeal
- retrorocket
- retrorse
- retrorsely
- retrospect
- retrospection
- retrospective