The fruit of certain woody plants. There are two types: hard mast and soft mast.

Examples of hard mast producers are oak, hickory, beech, walnut, hazelnut, pecan
Examples of soft mast producers are black cherry, wild grape, sassafras, hawthorn, mulberry, sumac, flowering dogwood

Mast crop production varies by year and by species and is highly unpredictable for some producer species, especially the white oak group, red oak group and beech group. Insect infestations can damage up to 90 percent of a hard mast crop in bad years. Hard mast production varies more widely than soft mast production. Factors affecting mast crop production include light intensity, rainfall, temperature fluctuations.