Invulnerable Attack

Ripped shirts also make for a surprisingly effective combat bonus.

Everyone knows That One Boss is so hard (among many other reasons) because he's only vulnerable for five seconds out of every sixty. Won't he be pleasantly surprised when he realizes the Player Character has learned to do this too!

There are two kinds of Invulnerable Attack; the first happens in games that have a Charged Attack mechanic, and while it's charging the player is invulnerable to enemy attacks (though he might still suffer knockback). The second is when the charged attack executes, so while the overly long CGI plays out the player can take a nuke to the face without flinching.

The aversion is pretty common though, that being hit during the charge up or execution to the attack does cause damage and may even interrupt it altogether. Usually it's far, far more common for a charged attack to leave you motionless and vulnerable - this is usually an intentional counterbalance to charge up attacks being more powerful.

A savvy player can use this to "block" otherwise unblockable and unavoidable area attacks. Even if the boss is also immune to the charged attack during his own "unavoidable attack", the player can save a boatload of health. Similarly, part of beating most bosses involved players learning to dodge or avoid the parts of their attack pattern where they're invulnerable.

Contrast Mercy Invincibility. May result during a Meditation Powerup. See also Unblockable Attack.

Examples of Invulnerable Attack include:

Invulnerable charge ups

Anime and Manga

In the original Slayers Light Novels, Lina describes how a magic user is protected by a weak forcefield while chanting a spell - it's not usually powerful enough to repel an actually dangerous attack but it helps a little. When she sees Xelloss' field deflect an incoming fireball she knows that she is dealing with someone who is really quite powerful. His amplification trinkets don't hurt, mind, and the resultant spell is... very messy for the victim. In the later anime series, Slayers Revolution and Slayers Evolution-R, we see Lina's forcefield deflecting non-trivial attacks while she's casting.

Video Games

RPG

Pokémon has a variety of moves ("Dig", "Fly", "Dive", and "Bounce") that provide one turn of invulnerability to most attacks before striking the opponent on the next turn. This is moderately useful in single player, but less so in competitive multiplayer where a Genre Savvy opponent knows how to quickly counter or defend against the incoming strike.
- On the other hand, "Focus Punch" is a very powerful attack, but strikes last in a turn and will fail to do anything if the user takes damage from the opponent's attack.
Jumping in many Final Fantasy games makes the user invulnerable to all attacks for a while, then letting them deliver Death From Above (and deals more damage with a Spear) as they drop back in.

Action Games

If it's a charge-up attack, the protagonist of The Force Unleashed is basically untouchable while charging up his Force Maelstrom power. Very handy if (as the game does) you have to take out an End of Level boss and he's got some Mooks around to help him.
In Dynasty Warriors: Gundam, enemy pilots don't suffer knockback when they're charging up for a Musou attack (typically only used when the pilot's health is more than halfway depleted).

Platformer

Prinny: Can I Really Be the Hero? has this, except you're invincible while charging, and it's not an attack: it's a dash. (Though you can segue from the dash to an invincible slide.)
Sonic the Hedgehog's spindash charge in the original games left him protected from enemy Collision Damage. He was still vulnerable to attacks, spikes, projectiles, etc., so not completely invincible.
Iji's Retribution move.