noun Horology.
a fine, usually spiral, spring used for oscillating the balance of a timepiece.
Also called balance spring.

1820–30; hair + spring Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
hairspring (ˈhɛəˌsprɪŋ)
horology a very fine spiral spring in some timepieces, which, in combination with the balance wheel, controls the timekeeping

Collins English Dictionary - Complete & Unabridged 10th Edition
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