A score of one stroke under par on a single hole is called a birdie.
The term describes the relationship between a golfer's score and the hole's par. On a par-three hole, completing the hole in two strokes is a birdie; on a par-four, it takes three strokes; and on a par-five, it takes four. The score is recorded as −1 for that hole.
The word is generally traced to the Atlantic City Country Club in New Jersey around 1899. A popular account says that golfer Ab Smith called an excellent shot “a bird,” and the group began using “birdie” for finishing one under par. The expression spread through American golf and became standard terminology.
A birdie is not the same as an eagle, which is two strokes under par, or a bogey, which is one stroke over par. A golfer who makes birdies on all 18 holes would shoot 18 under par, often described as a perfect round, although that feat has not been achieved in a professional tournament.