Outdated MTG phrasing which is still in use, either mistakenly, colloquially, or historically.
In Magic: The Gathering, mana value began life as casting cost or total casting cost prior to Sixth Edition. After the Sixth Edition rule updates, it grew into converted mana cost (or CMC). The term "mana value" wasn't introduced until Strixhaven: School of Mages. Afterward, all cards previously printed have had their oracle text updated to reflect the change.
For a little tech speak, the mana value of an object is an integer and is always greater than or equal to zero. For more, please see section 601 of the comprehensive rules.
Mana cost, on the other hand, when used technically (on card text for example), means the default cost to cast a spell as defined on its card without applying alternative costs, cost reducers, or additional costs, amongst other things. This property of spells was once called casting cost as well. The mana cost of an object typically determines not only that object's color but also its mana value, although they are not the same thing as far as the game is concerned.
Mana cost is a characteristic found in the upper-right corner of most cards, although variants and special frames don't always follow this rule. It is exactly all of the symbols printed thereon, including color. When a mana cost includes the symbol "X", X is equal to the announced or defined value only while the spell is on the stack, unresolved. X is zero elsewhere, e.g. library, hand, graveyard, exile, et cetera.
Other such word changes are tribal to either typal or kindred, not to mention Elder Dragon Highlander, or EDH, to Commander.
Pro tip: Always, always, always check the oracle text, especially when confronted with the phrase, "reading the card explains the card."
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