Over/under 2.5 is a goals market. It asks whether both teams' goals add up to three or more, and it does not matter who scores them: 3-0 and 1-2 both settle as over.
Over 2.5 means three goals or more; under 2.5 means none, one or two. Because no match can finish on exactly 2.5 goals, every fixture settles cleanly one way or the other — no push, no void. The 2.5 line is used because it sits close to the typical top-European-league average of roughly 2.4 to 2.8 goals a match, which keeps both sides of the market competitive.
Bookmakers also price 0.5, 1.5 and 3.5. Lower lines land more often at shorter prices; higher lines land less often at longer ones. Historically about half of top-five-league matches go over 2.5, and roughly a third go over 3.5.
The form and style of both sides, the head-to-head history, the scoring culture of the league, the context of the match, and absences among the key attackers or defenders. We publish our own over-2.5 probability next to the bookmakers' number with the margin removed, and grade it against the closing line. A probability is not a tip. 18+, and please gamble responsibly.