Underdog
The team the betting market considers less likely to win a game.
The underdog is the team not favored, usually shown with a positive spread (for example, "+4.5"). In effect, that team is given a virtual head start in points to balance perception of the matchup.
Underdog status says nothing about a team's actual quality — only about how it's evaluated relative to its opponent in one specific game. Plenty of underdogs are strong teams simply facing an even stronger one.
Games where the underdog wins outright, not just "covers" the spread, tend to be among the most talked-about moments of a season.