A mast year is a year in which vegetation produces a significant abundance of fruit. The term originally applied solely to trees, like oak trees, that produce fruit useful for feeding farm animals. The term "mast" comes from the Old English word "maest", meaning the nuts of forest trees that have accumulated on the ground, especially those used as food for fattening swine.
More generally, masting describes not only abundance, but also paucity of fruit production, so masting is a group phenomenon that results when plants within a population correlate their reproductive activity both in time and in size of crop. It is thought that masting occurs as an evolutionarily stable strategy enabling plants to exert some influence over the size of animal populations that predate on the fruit.