The Greek word "gnosis" means "knowledge," but in "gnosticism" it is used as "revealed knowledge." The label "gnosticism" is used for a variety of movements, usually, though not necessarily, of Christian orientation, which share common ideas.

Central beliefs:

Gnosticism flourished in the 2nd and 3rd century AD. It was heavily nfluenced by Greek philosophy (Plato's dualism) and Zoroastrianism (fight of Light / Good against Dark / Evil).
In turn, gnostic thinking influenced the Cabala, Theosophy, as well as some Christian sects like the Albigenses, Manichaeism ...

(Trivia: Some of the scrolls found in 1945 in Nag Hammadi, Egypt, contained Gnostic texts, whereas others contained writings from the very early Christians. Some texts are even attributed to some of the apostles!).