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, the heavens or spiritual universe. More on The Pleroma
The Greek word for "fulness" used by the Gnostics to mean the highest principle of Being where dwells the unknown and unknowable God. Used in the New Testament to refer to "fulness in Christ" (Col. 2:10) who is the known revelation of God in the flesh.