Wikipedia has a pretty good article on this.

Long story short: in 1795, it was defined as 1⁄10,000,000th of a half-meridian. From 1799 until 1960, it was defined through a series of carefully machined Platinum-iridium bars.

When that definition proved to be too vague / problematic, it was redefined using the physical properties of krypton gas. And In 1983, it was redefined again to be the light travels in a very short period of time.