This article is from the Magic: The Gathering Rules FAQ, by Patrik Linell (pls@claymore.nu) with numerous contributions by others.
The total amount of mana in its mana cost, printed in the top
right corner of the card. For example, Devouring Strossus (cost 5BBB) has a
converted mana cost of 8. Even if the Strossus-toting player has a couple
of Thunderscape Familiars out, the converted mana cost is still 8; it never
changes. You only have to pay 3BBB to play Devouring Strossus with two
Thunderscape Familiars out, but Monkey Cage will still produce 8 Apes when
it comes into play.
 
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