The mana value (also known as converted mana cost [CMC] from Sixth Edition to Time Spiral Remastered, and casting cost in sets prior to Sixth Edition) of an object is an integer greater than or equal to zero. It's a number that determines the quantity of mana in a mana cost.
Rules
From the glossary of the Comprehensive Rules (April 12, 2024—Outlaws of Thunder Junction)
- Converted Mana Cost (Obsolete)
- An obsolete term for mana value. Cards printed with this term have received errata in the Oracle card reference.
It is determined by converting each colored mana symbol in the spell's mana cost to 1 (unless it is one of the hybrid mana symbols 
, each of which has a value of 2), then adding the results to the generic mana cost of the spell. (For example, spells with mana costs of 
and 
both have a Mana Value of 3.)
The only case in which a spell's converted mana cost can ever vary is for spells with 
in the mana cost. When an object with X in the mana cost is on the stack, X equals whatever value was chosen for it when it was put on the stack. In any other location, X equals 0.
Odd or even mana cost
Some cards care about if the converted mana cost is odd or even.
- Ashling's Prerogative (Lorwyn)*
- Void Winnower (Battle for Zendikar)*
- Extinction Event (Ikoria: Lair of Behemoths)*
- Gyruda, Doom of Depths (Ikoria: Lair of Behemoths)
- Lavabrink Venturer (Ikoria: Lair of Behemoths)*
- Obosh, the Preypiercer (Ikoria: Lair of Behemoths)
^* Features the reminder text: zero is even.
Cost matters
- Scryfall search: oracle:"converted mana cost"
"Cost matters" was a theme in Scourge that encouraged players to use spells with high converted mana costs. In addition to providing numerous creatures and other spells with converted mana cost 6 or greater, several cards were printed that enhanced or facilitated them.