Generic mana cost

A generic mana cost is a mana cost that can be paid with mana of any type; meaning mana of any color, as well as colorless mana.

A generic mana cost is represented by a number or an X in a gray circle (, , , , , and so on).

Rules

From the Comprehensive Rules (April 12, 2024—Outlaws of Thunder Junction)

• 107.4b Numerical symbols (such as {1}) and variable symbols (such as {X}) represent generic mana in costs. Generic mana in costs can be paid with any type of mana. For more information about {X}, see rule 107.3.

From the glossary of the Comprehensive Rules (April 12, 2024—Outlaws of Thunder Junction)

Generic Mana

Mana in a cost represented by numerical symbols (such as {1}) or variable symbols (such as {X}) that can be paid with mana of any type. See rule 107.4.

Difference with colorless

There is no such thing as "generic mana". Though you can use colorless mana to pay for generic mana costs, they are not the same thing. "Generic" is a type of cost. "Colorless" is a type of mana. You can't produce generic mana, but you can produce colorless mana and have a colorless cost (first introduced with Oath of the Gatewatch).

R&D came to the conclusion that they had to stop using the mana circle with a number to represent both generic and colorless mana. Changing the representation of colorless mana was easier, as it's represented far less, so they to made the new mana symbol . This change was made along with the rules update for the expansion Oath of the Gatewatch, which features colorless as well as generic mana costs.

From the Comprehensive Rules (April 12, 2024—Outlaws of Thunder Junction)

• 106.10. If an effect would add mana represented by a generic mana symbol to a player’s mana pool, that much colorless mana is added to that player’s mana pool.