What does the “64” in Nintendo 64 refer to?

The “64” in Nintendo 64 refers to its 64-bit processor.

Nintendo released the console in Japan in 1996, followed by North American and European launches. Its name highlighted the system's 64-bit central processing unit, a technical feature intended to distinguish it from earlier 16-bit and 32-bit consoles.

The Nintendo 64 used cartridges rather than the optical discs adopted by some competing systems. Cartridges offered fast loading and made copying more difficult, but they generally provided less storage capacity than CDs and contributed to higher manufacturing costs.

The number does not mean that the console held 64 games, had 64 megabytes of memory, or automatically supported 64 players. Although some Nintendo 64 games offered multiplayer for four people and accessories expanded certain possibilities, those features are unrelated to the console's name.