The scientific study of cyclic and seasonal natural phenomena, particularly the timing of biological events such as flowering, migration, and hibernation.

From the Greek phainein (to show, to appear) and logos (study) — the study of appearances, particularly of nature through the seasons.

Of increasing importance to climate science, as shifts in phenological timing — such as earlier spring flowering — serve as sensitive indicators of climate change.

Robert Marsham's records of twenty-seven recurring natural events on his Norfolk estate, kept from 1736 onwards, are considered the first systematic phenological dataset in British science.

Gilbert White's Natural History of Selborne contains the word phenology in its first chapter, making it the earliest printed use of the term in English natural history writing.