i think the entire term is a piece.
let's see...
"sports"
plural form of "sport"
sport:
sport Audio pronunciation of "sport" ( P ) Pronunciation Key (spôrt, sprt)
n.
1.
1. Physical activity that is governed by a set of rules or customs and often engaged in competitively.
2. A particular form of this activity.
2. An activity involving physical exertion and skill that is governed by a set of rules or customs and often undertaken competitively.
3. An active pastime; recreation.
4.
1. Mockery; jest: He made sport of his own looks.
2. An object of mockery, jest, or play: treated our interests as sport.
3. A joking mood or attitude: She made the remark in sport.
5.
1. One known for the manner of one's acceptance of rules, especially of a game, or of a difficult situation: a poor sport.
2. Informal. One who accepts rules or difficult situations well.
3. Informal. A pleasant companion: was a real sport during the trip.
6. Informal.
1. A person who lives a jolly, extravagant life.
2. A gambler at sporting events.
7. Biology. An organism that shows a marked change from the normal type or parent stock, typically as a result of mutation.
8. Maine. See summercater. See Regional Note at summercater.
9. Obsolete. Amorous dalliance; lovemaking.
ok, so lets take def #1.
"Physical activity that is governed by a set of rules or customs and often engaged in competitively."
Therefore, In order to be fully classified as a sports car, it needs to be on a list of some sanctioned competition.
thus, IMO, a sport car is something listed on this website:
http://www.moutons.org/sccasolo/Lists/2005/stockm.html