A person who wagers or bets money on the outcome of an uncertain event, such as a sporting event or race.
From 'bet' (verb) plus the agent suffix '-or' (sometimes '-er'), which creates nouns for people who perform an action. This follows the English pattern of actor, creator, bettor.
The spelling 'bettor' (not 'better') is a linguistic quirk that exists because English speakers needed to distinguish between people who bet and things that are better—a homograph collision solved by preserving the original '-or' agent suffix rather than using '-er'.
Complete word intelligence in one call. Free tier — 50 lookups/day.