A prime is a creature born on the Prime Material plane. Often called Outsiders, or more derogatorily, "Clueless," although that depends on the prime. Getting to the planes generally requires a certain degree of power, even if they have some strange ideas in their heads, so primes are afforded some amount of respect.

Most primes encountered on the planes are adventurers, there for a specific task, and will likely return home once their job's done. Others are trapped, or take a liking to their new surroundings, and set up a permanent home in the planes.

Primes have some qualities that planars do not:

• Primes have a silver cord in the Astral Plane, tracing their way back to their bodies left behind on the Prime Material.
• Primes are completely immune to planar magic, like protection from evil or monster summonings.

"Prime" is also sometimes used as a shorthand for the Prime Material Plane, but in such cases, the word is capitalized: e.g. "a prime comes from the Prime."

References

• Planescape: Planescape Campaign Setting, p. 8 , A Player's Guide to the Planes