Invincibility

An invincibility frame is a period of time where a character is completely invulnerable to damage, grabs, and knockback. Invincibility frames are not to be confused with knockback resistance, which only reduces or negates the effect of knockback — not damage — from an attack, or intangibility frames, where a player is, essentially, invincible, however will not receive hitlag if on the receiving end of an attack.

Characters can gain invincibility frames in a few ways. An example can be during the initial stages of a character entering their Final Form or using some other form of a Final Smash, which grants them invincibility frames as they transform or execute their attack to prevent interruption.

Intangibility
A character with intangibility will not be hit by attacks. For example, a projectile will travel right through them. Intangibility is often attributed to dodges, techs, edge-grabbing, floor attacks and animations, and certain body parts during certain moves, such as 's up smash. Since the character is not hit, a long-lasting attack (or very short intangibility time) may still hit the character once it ends. Intangibility can also be called "dodging invincibility".

Trivia

 * In the original Super Smash Flash, the only time characters get invincibility frames is when they spawn back to the stage after getting KO'ed. Otherwise, they are practically non-existent.
 * Because of how invincibility frames are currently treated in Super Smash Flash 2, a character under them will be unable to get a Super Star, a Super Mushroom or Poison Mushroom event if they were directly touched. This flaw is more evident if the invincibility was granted by a Super Star itself. This makes it possible to attack the hitboxes of the aforementioned items, making possible to launch them.