Prime number
From Memory Alpha, the free Star Trek reference
A prime number is a number with a certain universally recognizable characteristic. Two, three, five, seven, eleven, and thirteen are prime numbers.
When captured by unknown aliens, 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. (VOY: "Blink of an Eye")
