Under
"Under" refers to a result under par — you needed fewer strokes than par.
"Under" is the most important word of score language. A golfer playing "two under" lies two strokes under par. The figure can refer to a hole, to nine holes or to the whole round. On the leaderboard of the professionals the best rounds with a minus sign are right at the top, such as "-5". The counterpart is "over", and exactly in between lies Even, meaning par played. Anyone who plays individual holes under par has made a birdie or better.
A concrete example: the course is par 72. You come into the clubhouse with 70 strokes and say: "I played two under." Intermediate scores work the same way — after nine holes on a par-36 stretch you stand at 34 strokes, "two under". In the tournament report you then read: "After the first round he is three under par and leads." The way of counting is uniform internationally.
A widespread misconception concerns the sign: minus is good in golf. In most sports a negative result stands for something bad — here it is the goal. Equally important is the distinction between gross and net. With your handicap offset you can play "net under par" even though your gross score is over par. That explains why in Stableford amateur golfers regularly post results "under par" as well.
FAQ
Why is a minus result a good thing in golf?
Because golf counts backwards: the fewer strokes, the better. Anyone playing "two under" needed two strokes fewer than par requires, and on the leaderboard the minus values are right at the top. In most other sports a negative result means something bad — here it is the goal.
Can you as an amateur golfer also play "under par"?
Gross, that is difficult for most; net, it is completely normal. Offset against your handicap, your result can be under par even though your actual stroke count is above it. That is exactly why amateur golfers regularly post results in Stableford that feel like "under par". The way of counting remains uniform internationally.
Does "under" always refer to the whole round?
No, the figure works on every level. You can play a single hole under par — then you have made a birdie or better. Just as well you can stand "two under" after nine holes or finish the whole round with 70 on a par-72 course as "two under". The context follows from the conversation or the display.