One of the most common definitions is "a 2 seater". But, let's analyze the word "sportscar". It is simply a SPORT CAR. The CAR SPORT being racing. Therefore, a sportscar must be competitive in "stock" "production" racing. Unfortunately, "stock" and "production" are terms that are often severely abused in racing.
The only way to truly rate sportscars is to take brand new TOTALLY UNMODIFIED PRODUCTION cars and pit them against each other at the dragstrip, racetrack, and rally circuit. Don't compare them by horsepower or engine size -- totally irrelevant! They should be categorized by Manufacturer Guaranteed Dealership PRICE, such as:
The Mustang, WRX, and Corvette are great examples of sportscars, offering excellent overall performance, especially relative to their price ranges and cost based competition.