**G** or **g** may refer to:

- Graham's number, a famous large number invented by Ronald Graham
- G function, a function by Jonathan Bowers, used as G(
*a*) with the base of*b* - Ackermann's Generalized Exponential Notation
- Joyce's More Generalized Exponential Notation, an extension of the above

- Graham Array Notation
- Slow-growing hierarchy, a certain hierarchy mapping ordinals \(\alpha\) to functions \(g_\alpha: \mathbb{N} \rightarrow \mathbb{N}\)
- G(
*n*), the number of steps it takes for the Goodstein sequence starting on*n*to reach zero - g(n), the weaker cousin of the Goodstein sequence
- The symbol for the SI prefix giga-