A person, typically a sportsbook employee, who sets betting lines for an event.
A linemaker (a.k.a. oddsmaker) will adjust his or her opening line as bets come in. Their goal is generally to attract an even amount of bets on each side of the line. So their lines must be accurate.
The top Vegas linemakers have the best statistics, trend data, backtesting and technology at their disposal. And they’re masters of sports bettor psychology — i.e., they know all the public’s biases. That leads to very precise lines the overwhelming majority of the time.