A season ticket, commonly, is a ticket (or tickets) good for several trips to an attraction, limited either by number or by time. This type of ticket is normally sold at a discount as compared to the full price per event or visit.
Season tickets are many times sold for sports team home games, concert series, amusement parks, and other venues where the consumer would want to visit more than once, but not at the full price. They are a mild form of price discrimination, as the seller attempts to find the maximum price point at which the consumer is willing to buy.
Occasionally, the price for an entire season of events costs as much or more than the individual tickets would. This is a function of demand. The Green Bay Packers, a professional football team, have an extensive waiting list for those wishing to buy season tickets that there is no discount for buying a full slate.
My experience has been that amusement parks and other venues with unchanging or slow-changing attractions have season tickets which are 'good' for as many visits over a specific timeframe (the summer, January, etc) as the consumer wants to use, while concert halls and sports stadiums sell season tickets which are packaged around specific dates and events. In the first sense, season ticket is interchangeable with season pass. In the second, ticket package is occasionally heard.
Another synonym for season ticket is commutation ticket; this is nearly always used in the sense of a ticket usable on the railroad or other form of transportation, as opposed to the more general usage above.