The blazer is a type of men's jacket of military derivation and reached its peak popularity as a uniform of famous British colleges and clubs. For this reason, it was born as a garment with its own identity, not necessarily as part of a suit, and with a less rigid and formal style.
Blazers, however, are dressier than sportcoats but not as formal as suit jackets. Traditionally, a blazer is wool, blue with a pocket, and has distinct silver or gold buttons. However, this "tradition" has receded over time, with men opting for cotton varieties and blazers without the customary buttons.