Gnosticism: An early Greek religious movement of broad proportions that was particularly influential in the second-century church. Many biblical interpreters see in certain NT documents (such as 1 John) the attempt to answer or refute Gnostic teaching. The word gnosticism comes from the Greek term gnosis, meaning “knowledge.” Gnostics believed that devotees had gained a special kind of spiritual enlightenment, through which they had attained a secret or higher level of knowledge not accessible to the uninitiated. Gnostics also tended to emphasize the spiritual realm over the material, often claiming that the material realm is evil and hence to be escaped.1
You may also be interested in this lecture: Gnosticism Past & Present.
1. Stanley J. Grenz, David Guretzki & Cherith Fee Nordling, Pocket Dictionary of Theological Terms (Downers Grove, IL: InterVarsity Press, 1999), p. 56.