A prime number is a number with a certain universally recognizable characteristic.
When captured by unknown aliens in 2366, Captain Jean-Luc Picard signaled the first six prime numbers by pushing the same button in sequence, hoping to prove to his captors that he was intelligent. (TNG: "Allegiance")
Astronomers on Kelemane's planet also signaled prime numbers to the "Sky Ship" in an effort to communicate. Their attempts were successful. (VOY: "Blink of an Eye")
A prime number is a positive integer that is only divisible by itself and 1. One prime of particular note to the Star Trek franchise is 47.