Most people use the terms blazer, sport coat, jacket, and suit coat interchangeably. In fact, as I was trying to find clear distinctions, I came across different definitions from various reputable on-line sources. Here’s what I’ve surmised:
Like in biological classification Jacket is the order. It’s a tailored article of clothing with sleeves, a collar, and 2 or 3 buttoned up from the waist. It’s worn over a shirt and is distinct from a waistcoat, overcoat, and mackinaw. It can have different arrangements of pockets and vents (the slits in the back). But the overall shape and usage is pretty consistent for the two main families.
Those would be Suit Jacket and Odd Jacket. The difference is that the former has a matching pair of pants and perhaps a waistcoat. The genus Tuxedo Jacket fits under that classification. As well as the jacket of most military dress uniforms. While an odd jacket can be mixed with different pants.
Under the odd jacket family is the two major Genuses. Blazers and Sport Coats. Here’s where the confusion starts. Basically, a blazer is a solid color and has brass buttons. Originally styled after naval uniform jackets, they are primarily used in a semi-formal fashion. Often in conjunction with a school or private club, sometimes featuring a crest. But it’s very common for them to be generic. They’re more formal than a sport coat but less formal than a suit. A sport coat, on the other hand, generally features a pattern like houndstooth, plaid, stripes, checks, herringbone, etc. Or they can be made of a substantially textured material like tweed, corduroy or linen. A suit jacket and a sport coat may be very similar, but when worn with the rest of the suit they are considerably more formal.
More unique species of sport coat include shooting jackets, hacking jackets, continental jackets, smoking jackets, etc. The common thread is that they are meant for activity and are very much considered to be casual. For example, a navy blue jacket could be mistaken for any of the above, but clues like buttons, pants, material, and even the occasion will tell you whether it’s a suit coat, blazer of sport coat. Either way, a jacket is a must. They go a long way in distinguishing a gentleman.
Notice the level of formality that Jimmy exhibits with each jacket.