The word means "fullness". It refers to all existence beyond matter but not including Bythos who is beyond it. In other words it is the world of the Aeons and Archons and of our own spark, or spirit.

“Fullness”: the many-layered Gnostic heaven. Emanations of the ultimate hidden Godhood reach from heaven to earth. For the Gnostics, heaven is a state of abundance, whereas human existence tends to be characterized by alienation and exile.

The word means "fullness", and the "All". It refers to "all existence beyond matter. Refers to the world of the Aeons, the heavens or spiritual universe, which represents being out of the state of matter. According to the "Gospel of Truth" "....all the emanations from the Father are Pleromas". See Tractates 3, 2, Codices, I, and XII, Nag Hammadi Lib. Pleroma can have other connotations according to the Gnostic school of thought, some differences in Sethian and Valentinian (other) schools can be noted. Pleroma, is different than Logos. (See Logos, See aslo; Gaffney, p. 246.)