Invincibility

Invincibility is a state in the  series and Fraymakers in which a character is invulnerable to attacks. Attacks can still land on an invincible character and the attacker will still be affected by hitlag, but an invincible character will not receive damage, knockback, or hitstun. However, they are still vulnerable to taking recoil damage from any attack that deals it.

Characters can gain invincibility frames in a few ways, with the most common way being to revive after being KO'd. Certain attacks also grant temporary invincibility, making them safer to use when timed correctly. In Super Smash Flash 2, characters can gain invincibility by touching a Super Star or from winning a microgame in WarioWare, Inc., which lasts for an extended period of time.

Invincibility is not to be confused with knockback resistance, which only reduces or negates the effect of knockback — not damage — from an attack.

Intangibility
Intangibility, also known as dodging invincibility, is a state that is very similar to invincibility in that characters are invulnerable to attacks. The main difference, however, is that intangible characters cannot be hit by attacks at all. For instance, a projectile that normally stops on contact will travel right through them.

Intangibility is often attributed to defensive actions such as sidestepping, rolling, teching, or various ledge options, and it is also granted temporarily for various attacks. Since the character is not hit, a long-lasting attack — or a very short intangibility time — can cause a character to get hit regardless once the intangibility wears off.

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.
 * Due to how invincibility frames are currently treated in SSF2, a character under them will be unaffected by a Super Star, Super Mushroom, or Poison Mushroom when touched.