Defense can also refer to a character's options while in the middle of a combo, also called the disadvantage state. Because he's so big and he doesn't have any super fast movement options, a character like Bowser can be combo'd for a long while by a skilled opponent. Conversely, Mii Brawler has a move called Feint Jump that comes out fairly quickly and can leap them out of a combo. Thus, this Brawler would have a better disadvantage state than Bowser.
Then there's the matter of how good a character is at punishing opponents for approaching them. Bowser and Mario have up smashes that make them more or less invulnerable, making them very good at punishing opponents who try to attack from directly above them. These moves benefit from making the opponent come to you. The same can be said of Ganondorf's smash attacks--they excel at punishing aggressive gameplay.
Then there's a couple of weird cases like Yoshi's shield. Because it's an egg, it will always cover his whole body, as opposed to other characters whose shields will shrink and expose them over time. This is a defensive trait that is unique to Yoshi.
So to answer your question, it depends on the context. A character's defensive ability can mean all sorts of different things depending on how it's used in the guide.
