Down

Down is the "absolute" status effect in most RPG games. When a character's HP reaches zero, he or she falls in battle, then lays motionless on the ground. A downed party member cannot attack or use any commands. After a battle is won, any downed party members are restored to 1 HP (except Paper Mario: The Thousand-Year Door, where that HP remains zero), excluding bosses, when all party members are restored to full HP after being defeated.

Super Mario RPG: Legend of the Seven Stars
Party members are effectively out of the battle unless they are revived, either via a Pick Me Up or Princess Toadstool's Come Back technique. If all three party members are downed in battle, it's Game Over for the player.

Mario & Luigi series
When a brother's HP reaches zero, he falls in battle, and then is no longer able to battle until he is revived, via a 1-Up Mushroom, a 1-Up Super, or a 1-Up Deluxe.

Paper Mario: The Thousand-Year Door
If a partner's HP reaches zero with no Life Shrooms in possession, he or she falls in battle. They cannot act in battle or be revived for the rest of the battle and must be switched out.

A downed partner's HP is not recovered to 1 HP after a battle is won, excluding all bosses except Shadow Queen, when all partners are restored to full HP after falling in battle.

If Mario's HP reaches zero with no Life Shrooms, then he will be down and the battle is over as well as the game.