From AMS Glossary
An index calculated from air temperature to estimate fulfillment of plant dormancy requirements and the ability to start springtime growth, especially tree fruits.
Chill units are calculated in hourly time steps and summed over a time period. Cold temperatures add to chill units as the index ranges from 0 at temperatures less than 1°C, to 1 near 7°C, and 0 at 14°C. Temperatures above 14°C reduce chill unit accumulation as the index falls to -1 at 21°C. A minimum total number of chill units dependent upon species and variety must be exceeded before normal springtime growth can begin. Chill hour calculations frequently require hourly data from late evening to early morning.