Re: "Sports jacket"
I always thought this was an American thing - I have never heard anyone in the UK talk about one.
"Blazer" describes roughly the same thing in the UK, though the cultural histories are different.
What kind of jacket is suitable for doing sports in, I wonder?
The kind made for wearing while riding a horse. Some of the "sports" in question are things like fox-hunting and polo. That's why sports jackets have "vents" - slits that let the back of the jacket split around the horse's back (single vent) or sit on the horse's back without pulling the sides of the jacket up (double vent). The various nautical sports, particularly sailing, also once called for a sports jacket. Blazers in the UK were traditionally made in the colors of one's yachting club, so that one might be more easily cut by one's betters.
Vents eventually contaminated suit jackets, but if you look around you can find unvented suit jackets. Particularly recommended if you are not overly wide, and can wear a jacket with some decent tapering at the waist, rather than having to buy one that hangs straight down and makes you look like a barrel with legs.
(By the way, to those wondering about the difference between suit jackets and sports jackets: The short answer is that a suit jacket comes as part of a suit. Traditionally, suit jackets are more restricted in choice of fabric and style of cut than sports jackets, for the fairly obvious reason that the fabric also must be suitable for trousers and has to look of a piece with their construction. When sports jackets are made from a fabric that would be suitable for a suit jacket, they're often cut in a way that wouldn't go with suit pants, so that people can tell that you're not some barbarian wearing a suit jacket with the wrong pants.)