Today, the term “blazer” and “sport coat” are often used interchangeably, but historically there’s a difference. Let’s first discuss what separates a blazer from a sport coat.

Blazer vs Sport Coat
A sport coat is inherently more casual than a blazer, making it the least formal of the 3 jacket types. They are typically made of visibly textured fabrics like tweed or corduroy, while a blazer can be made of those fabrics as well.
Another distinguishing feature of a sport coat is its bolder patterns and colors, but a blazer can be bold and in a variety of colors too. Historically speaking, the sport coat was worn during “sporting” activities, hence the name. But since there’s little actual difference between the two, I just naturally default to saying blazer.
Blazer vs Suit Jacket
A suit jacket should always be worn with the pants that it was meant to be paired with, as it’s a mistake to wear a suit jacket with chinos or jeans. A blazer can be worn as a separate, but not a suit jacket. And a blazer can also be worn with denim or chinos, but a suit jacket should not.
The suit jacket is dressier than a blazer and is made of finer woolen fabric, whereas the blazer is often made heavier weighted fabrics. One final differentiator is that traditionally, the navy blue blazer has gold buttons, while suit jacket does not.
The bottom line:
Blazers/sport coats are more versatile than a suit jacket. Don’t make the mistake of wearing a suit jacket as a separate.
By Khoi Nguyen for Gent Within©
Source: How to Wear a Navy Blazer & Where to Buy It (Men’s Outfit Essential) (gentlemanwithin.com)