From AMS Glossary
An observational day on which frost occurs; one of a family of climatic indicators (e.g., thunderstorm day, rain day).
The definition is somewhat arbitrary, depending upon the accepted criteria for a frost observation. Thus, it may be 1) a day on which the minimum air temperature in the thermometer shelter falls below 0°C (32°F); 2) a day on which a deposit of white frost is observed on the ground; 3) in British usage, a day on which the minimum temperature at the level of the ground or on the tops of low, close-growing vegetation falls to -0.9°C (30.4°F) or below (also called a "day with ground frost"); and perhaps others. The present trend is to drop such terms in favor of something less ambiguous, such as "day with minimum temperature below 0°C (32°F)."