What is the spread in betting?

A handicap that turns a lopsided game into a coin flip: the favorite has to win by more than the spread number.

Favorite at −6.5 means they must win by 7 or more for your bet to land. The underdog at +6.5 "covers" even by losing, as long as it's by 6 or less. The line moves until both sides attract equal money, which is how a blowout becomes a fair bet.

A prediction market just asks the question directly, "wins by 7 or more?", and gives it its own percent. (Heads up: in the UK, "spread betting" also names a different real-money product where wins and losses scale with how far the result goes. Same words, much sharper teeth.)