TERM
Bookmaker margin
The share a bookmaker builds into its odds on top of fair probabilities: the sum of inverse odds on all outcomes at one book, minus one.
Odds of 1.90 and 1.90 on two equally likely outcomes give Σ 1/k = 1.053: a 5.3% margin. The lower a book's margin, the closer its odds are to fair and the more often it ends up as a leg of a surebet against high-margin books.
A surebet is a situation where two books price an event differently enough that the combined margin between them turns negative. Pinnacle's margin on top-tier esports is about 2–3%; many retail bookmakers run at 6–8%.