Pentarus III was the third planet of the Pentarus star system. The planet had two moons, including the "barely M-class" Lambda Paz (Pentarus III B). (TNG: "Final Mission")

"pen-TAR-us" was the pronunciation for Pentarus from the script pronunciation guide. [1]
According to Star Trek: Star Charts ("United Federation of Planets I"), in 2378, Pentarus III was listed as a Federation member.