Glossary odds
Over/Under (Totals)
In short: A bet on whether the combined score of both teams will be over or under a number set by the sportsbook. Independent of who wins or loses; only the total points/runs/goals matter.
Also known as: totals, O/U
Totals are often less efficient than sides because they get less attention from the betting public. This makes them fertile ground for finding mispricings, including middles where two books disagree on the line by enough to create a window where both bets win.