A precisely defined point in the life cycle of a plant or animal, generally marking the start or end point of a phenophase. The occurrence of a phenological event can be pinpointed to a single date and time (in theory, if not in practice). Examples include the opening of the first flower on a plant, the end of leaf fall on a tree, the first appearance of a particular songbird species in spring, or the start of fish migration up-river to spawn. (Note: The definition of the term “phenological event” has not yet been standardized and varies among scientists. The definition presented here reflects our usage of the term on the USA-NPN website.)