A Goa'uld's foremost Jaffa warrior, in command of his army and charged with carrying out his most important tasks. Their masters demand -- and reward -- absolute loyalty, as the First Prime is frequently tasked with passing along the Goa'uld's commands and making sure these are implemented.
Other named First Primes (and former First Primes) include Shak'l (who succeeded Teal'c as First Prime of Apophis), Herak (First Prime to Khonsu, and later Anubis), Oshu (in the service of Yu), Na'onak (Bynarr), Kytano (Imhotep), Ro'nac (Cronus), Cha'ra (Moloc), Trelak (Ares), Gerak (Montu), and Arkad (unknown).
Children of the Gods - Teal'c gives up his position as First Prime of Apophis and rebels against his enslaver, firing on his own men.
Threshold - In a dream-like state, Teal'c recalls his rise to the rank of First Prime under the careful tutelage of Bra'tac.
The Warrior - Kytano, who claims to have been first prime to the slain Goa'uld Imhotep, leads the rebel Jaffa -- until Teal'c reveals that Kytano is not Jaffa, but Imhotep himself.