发条是螺旋带状金属扭力弹簧，亦称发条弹簧，为手动机械表与某些时钟的动力来源。

早期的发条用碳钢合金制成，自1945年以来，已陆续被多种新的特殊合金（添加钴、钼、或铍的铁、镍、铬），以及冷轧合金（结构硬化）所取代。

A mainspring is a spiral torsion spring of metal ribbon - commonly spring steel - used as a power source in mechanical watches, some clocks, and other clockwork mechanisms. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it unwinds, until the next winding is needed. The adjectives wind-up and spring-powered refer to mechanisms powered by mainsprings, which also include kitchen timers, music boxes, wind-up toys and clockwork radios.