From AMS Glossary
A unit of measure for fulfillment of plant dormancy requirements prior to the start of springtime growth, especially tree fruits.
Determined by totaling the number of hours during a cold season when air temperature is below a specified value. The reference temperature (base temperature) for chill hour accumulation is species dependent; for example, for peach dormancy to be fulfilled, the number of hours that the air temperature is below 7°C (45°F) is accumulated. A minimum number of chill hours dependent upon species and variety must be exceeded before normal springtime growth can begin.