I found this as far as a "Definition" is concerned:
In the early second century a strange movement began to emerge, more strongly concentrated in Egypt, but with pockets of activity throughout the Roman world. Gnosticism was a curious synthesis of Jewish apocalypticism, Platonism, strains of pagan religions, and early Christianity. There are some indications of an early form of first century gnosticism in the NT, but nothing like what developed in the second century. Some scholars want to date various NT documents into the second century based on the apparent references to gnosticism.
To better understand Gnosticism, it is helpful to read the documents that represent the beliefs of Gnosticism. Our primary documents are found in a collection known as "The Nag Hammadi Library" (NH). The NH texts include The Gospel of Thomas, The Gospel to the Egyptians and many others. Some of these documents are actually more Christian than gnostic, but others are decidely gnostic, like "To the Egyptians." See a photo on the right showing a text from The Gospel to the Egyptians in the NH in English. The reader is supposed to sound out the long vowels sounds given in the text. To better understand Gnosticism in the ancient world, download the paper: A Brief Introduction to Gnostic Writings.
Similar to Marcion, basic Gnosticism consisted of an extreme dualism, drawing a distinction between the body and the spirit realm. The "demiurge" was the evil creator of the physical universe, humans were bound in their "evil" physical body, and could only be released from the confines of that body through the gaining of gnosis, or divine knowledge. The seven visible heavenly bodies (Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn) gave rise to a belief in eight heavenly realms. Plato had written about the concept of pre-existent souls in a state of perfection prior to taking on a mortal body on the earth. When the soul is released from the prison of the body it ascends back to the heavenly realm where it is reunited with the realm of ideas. The soul in the Gnostic system must ascend through these heavenly realms in the quest to return to a state of perfection. Along the way the soul must pass guardians of each level; typically to pass into the next stage, or heavenly realm, the soul must recite some of the heavenly gnosis learned during the earthly trek. The eighth level is the place of perfection, the ultimate goal for every soul.
Gnosticism in the second century was not a unified movement. Each group tended to gravitate around a single enlightened leader, and most groups were exclusive, seeing their particular set of dogma to be unique and essential. This lack of cohesiveness between Gnostic sects makes it difficult to quickly summarize the gnostic system beyond the above overview. To learn more, see the explanations regarding some of the chief gnostics of the second century: