A birdie is a score of one stroke under par on a single hole of golf. For example, taking three shots to finish a par-4 hole, or four shots on a par-5, is a birdie. On a scorecard, a birdie is often marked by circling that hole’s number.
Related scores follow the same pattern: an eagle is two under par on a hole, an albatross (also called a double eagle) is three under, and a bogey is one over par. The term birdie comes from American slang in which “bird” meant anything excellent; the United States Golf Association dates its first golf use to 1899 (BBC Sport).
Source: BBC Sport: What is a birdie, eagle and albatross? Golf terms explained
Image: Golf ball near hole, photo by Photos2Use, CC0, via Wikimedia Commons.