The real difference is that a suit jacket comes with a pair of matching pants. Some suit jackets lend themselves to independent wear, but many do not. For corroborating evidence, check out old Letterman shows. Dave tried to wear glen plaid suit jackets with odd trousers for a long time before kicking the habit. I often wear a navy blue suit jacket as a blazer. Lots of young, stylish actors and fashion designers favor wearing a slim, dark suit jacket with a pair of jeans. More casual suit jackets, such as corduroys or tweeds, are better candidates for moonlighting as separates than pin-striped ones are.