1. the principal spring in a mechanical watch or clock that provides the driving force
2. the chief motivating force or cause of something
Usage: figurative

Examples

• The watchmaker carefully wound the mainspring of the antique clock.
• When the mainspring broke, the watch stopped working completely.
• His desire for justice was the mainspring of his legal career.
• Economic necessity became the mainspring of their decision to move.
• The mainspring of the movement was a group of dedicated volunteers.
• She replaced the damaged mainspring with a new one from the repair kit.