A temperature inversion is where the temperature raises with an increase in altitude.
A temperature inversion often occurs over land on calm clear nights. The ground cools faster than the air. The cold ground then cools the air near the surface. As a result, the air temperature increases with height. A characteristic of an inversion is stable air. If the air was not stable, the air would be mixed and there would not be an inversion. An inversion often includes haze.
FAA AC-00-6B Aviation Weather pg. 2-12