From Glossary of Meteorology
The risk of damage by frost.
It may be expressed as the probability or frequency of killing frost on different dates during the growing season, or as the distribution of dates of the last killing frost of spring or the first of autumn. A strict application of the concept would take into account the actual species or group of plants that might potentially be killed, as different plants sustain frost damage at different temperatures. Wind-chill factors should also be considered.