X

X can mean two different things depending on its appearance on a card.

As a casting cost on a card, File:Manax.gif is a variable mana cost of a spell. In a card's text it means a variable, such as "Gain X life" or "Deal X damage" which is usually based on the File:Manax.gif in the mana cost of the spell.

From the ()

Examples

• Blaze - File:Manax.gifFile:Manar.gif
  Sorcery
  Blaze deals X damage to target creature or player.

Rulings

If a spell or ability has a cost with an “{X}” in it, the value of X must be announced as part of playing the spell or ability. (See rule 409, “Playing Spells and Activated Abilities.”) While the spell or ability is on the stack, the {X} in its mana cost equals the amount announced as part of playing the spell or ability. If a card in any other zone has {X} in its mana cost, the amount is treated as 0. If you’re playing a spell that has {X} in its mana cost and an effect lets you play it without paying any cost that includes X, the only legal choice for X is 0. This does not apply to effects that only reduce a cost, even if they reduce it to zero. See rule 409, “Playing Spells and Activated Abilities.”
If a cost associated with a special action, such as a suspend cost or a morph cost, has an “{X}” in it, the value of X is chosen by the player taking the special action as he or she pays that cost.
In triggered abilities, X is defined when the ability resolves. It may be defined by the text of the ability, by a keyword ability of the card, or by the trigger event. See rule 410, “Handling Triggered Abilities.”
In other cases, X is defined by the text of a spell or ability. If X isn’t defined, the controller of the spell or ability chooses the value of X. All Xs on an object have the same value.