Defense
Defense refers to the actions taken, or not taken, when the opponent is in position to score and is approaching that outcome. Using other spectator sports as reference, in basketball, the team guarding their hoop to prevent the opponent from making a basket and earning a point.
In fighting games specifically, the "score" is a round taken by hitting your opponent until their health reaches zero. Being in a defensive position means blocking as many hits as possible until the opponent finishes their attack string with an unsafe option.
Some famous mechanics, such as Ryu's EX Shoryuken or Sol Badguy's Volcanic Viper, act as invulnerable defensive options immediately available after an opponent's gap in their pressure. These will come out on the first available frame and will beat strike options.
What is Defense in Fighting Games?
How is Defense Balanced?
Because characters can have powerful and fun abilities attached to their bipedal (most of the time) kits, it can be difficult to keep defense in check when creating a plethora of offensive options. That being said, certain character designs emphasize the push-pull identity of strong offenses versus strong defenses. For example, a character with a hyper-offensive kit that is near unstoppable once they get in would be way too strong if they had the defensive capabilities to return to their offensive state out of defense with ease. Conversely, characters who tend to have weaker overall hits and shorter offensive pressure tends to be advantaged on defense for whatever reason. Axl Low has full screen buttons to react with despite his poor offense.