There is a consensus communis regarding what Gnosis ( as different from Gnosticism, a historical movement/religion ) is. I cannot back up my contentions ( I don't have here a few books illustrating the thesis, so, I'm writing from memory ). In short: the term "Gnosis" has come into wider usage from circa mid 19th century, having become more and more clearly defined after the discovery of the Dead Sea scrolls and, generally, on many meetings of professionals in various fields ( anthropology, comparative religion, religious psychology....) after WW2 ( congresses in Bologna and other academic centres ). After numerous discussions, a pretty unanimous definition has emerged, covering a multitude of worldviews ( Christian Gnosticism, Aztec religion(s), Cathars, Manichaeism,..). The summary can be put in three points:
1. The essential prerequisite for Gnosis, its fundamental postulate is the recognition of "divine spark" within, ie, an individual "immortal entity" that is not of "this world", nor a part of empirical psyche. This "spark" has many names: the inner man, the best and oldest self, the deepest self, the true "I", divine spirit within, angel, the ancient being, the immortal self, daemon,.. In sum, it is supposed to be the "authentic self", while empirical, existential, temporal psyche is a "false I" ( or one of the multiplicity of "I"s ).
There is no consensus re the nature of this "spark": some hold that it is equal to some acosmic source ( at least in nature ); some consider it to be perpetually awake; the others, on the other hand, deem it deluded, "asleep:, etc. Also, some consider this "spark" to have become enmeshed in the Creation-Fall scheme; for others, it remains always transtemporal/transcosmic and free. Needless to say, many religions ( orthodox and their heterodox offshoots ) don't have "divine element within" ( Christianity, alchemical Taoism, Theravada Buddhism. )
2. The second thesis is on the nature of the world or cosmos. The prerequisite for a doctrine to be considered gnostic is the admission of inadequacy or "unauthenticity" of the world human beings live in. In stark difference to many orthodox religions ( or secular worldviews ), Gnosis holds that humans' "natural" state in this world is that of contingency, dissatisfaction, bondage and exile. This stance can also cover multifarious "theosophies": world can be seen as "evil", or illusory, or semi-illusory, or a battleground, the field of exile, but also of redemption,..etc. In sum: the keyword would be "dissatisfaction". At best, human beings, confined and enclosed within gnostic "box" of the empirical natural world, can live lives of temporary relief and fleeting pleasures. True peace and fulfillment evade them.
3. The third component of gnostic worldview would be the answer to the question: "What is to be done ? How can we be saved ?" The usual ( and misleading ) answer was modelled on Christian Gnosticism: by contemplation ( theoria ), not by faith ( orthodox Catholic Christianity ).
However, as our knowledge, information and stances changed, a tad fuzzy answer emerged: the aim of gnostic ( whichever affiliation ) salvation, redemption, liberation and fulfillment is return to the luminous transcosmic world. The means include a variety of practices: contemplation, prayer, laying of hands ( shaktipat ), rituals ( both ceremonial and initiatory, Hermetic- and Shamanism-wise ).
Also, there is no clear aim what a liberated "spark" may do after its release from bondage: it may rest in the "enlightened" state, virtually passive; it can choose to return to the temporal and created world in order to help others to attain salvation. This point remains equivocal.
What are chief differences between Gnosis and other "esoteric" or spiritual Weltanschauungen ? Just to name a few: