I tend not to like to use the term “gnosticism” in casual conversation, because it can refer to a number of different things, and so is often more likely to lead to confusion than to clarification when it is used.
But even though the term’s definition varies pretty wildly, I don’t think that it is entirely useless. First of all, as with most terms, we should look to the literal meaning of the word, as derived from its etymology. Gnostic is derived from the Greek gnostikos (γνωστῐκός), itself from gnosis (γνῶσῐς), meaning knowledge or insight. Gnostikos simply applies the adjectival suffix -tikos (-τικός), so we have a word here which just means, effectively, “of or relating to knowledge.” But gnosis was not the only Greek word for knowledge; we have also episteme (ἐπιστήμη)*. The precise distinction between the two varies depending on period, but basically episteme refers to knowledge gained through a process of reasoning, while gnosis derives from direct experience and observation. In later periods gnosis also develops a connotation of knowledge of those things which are hidden, esoteric.
I believe we can basically point to two conceptual clusters which are referred to when the term “gnosticism” is used, both of which derive from the concept of gnosis.
The first sense seems to be more common among academics writing about religion, and is the more specific usage of the term. In this sense gnosticism refers to a particular cluster of beliefs and practices which emerge in the first and second centuries in the Hellenistic world. They emphasize hidden knowledge and esoteric practices, are likely to place a strong emphasis on a division between the spiritual realm and the physical realm, and are likely to identify the creator of the world (the demiurge [δημιουργός]) as being somehow separate from or secondary to God**. These premises are then often taken to mean that it is proper to seek a return to the spiritual world and away from the material world in which there is great suffering, that we must somehow seek a connection to God, and reject veneration of the demiurge. This sense of the word is codified in various encyclopedia entries on gnosticism.
This first sense of the term is used most centrally to refer to a collection of Christian belief systems, ultimately deemed heretical by proto-orthodox*** Christians. But of course, like most categories, this term refers to a cluster of related things with more and less central examples. Certain clearly non-Christian texts, such as the Corpus Hermeticum, which discusses a demiurge and a spiritaul Father, are part of the same intellectual milieu that produced the various texts held as central examples of gnosticism. So, also, some Jewish traditions in the Second Temple period point towards the idea that there was a second power in Heaven, responsible for creation and maintenance of the world and separate from the transcendent God.
So even this first usage of the term gnosticism, as referring to a particular lineage that emerges in a particular time and place, points at a concept which is broader than just the Christian traditions usually identified as gnostic. But there is another fairly common way that the term is used, which points to a concept broader still.
This second sense is more defined in terms of the literal meaning of the word. Rather than talking about a specific grouping of traditions, some people - particularly practicing esotericists - use the term to refer to any tradition which places emphasis on hidden, revelatory knowledge. See, for example, The Gnosis Archive.
This sense is obviously much broader than the first. This is the sense in which the term is typically used in Western esoteric and magical traditions, referring to any practice characterized by revelatory or initiatory knowledge****. If a lodge or other organization is referring to itself as “gnostic,” this is probably what they mean. But in this broader sense, there is a sort-of gnosticism present even within mainstream Christianity, present within the mystical works of figures like Meister Eckhart and Hildegard von Bingen - though mainstream Christianity is careful to contain this mystical knowledge within the boundaries of the faith that have been established through reason, episteme.
It’s worth noting also that, in addition to the obvious use of the term to refer to Western traditions, one can easily find instances of this broader sense of gnosticism in non-Western traditions. The Dharmic religions, with their often pluralistic nature, have plenty of room for gnosis, though the most obvious example is tantra.
Most miscommunications that I see happen between people using the term “gnostic” come from cases where one person is referring to one of these two concepts, and the other person the other, and each of them thinks that the other is using the term in an incoherent way. I believe that separating out these concepts will help to avoid such miscommunication, and propose the terms “local gnosticism” and “global gnosticism” to refer to the two different concepts.
*Actually, there are more than two Greek words which could be translated as “knowledge.” But contrasting it with episteme should be sufficient to point to what specifically gnosis refers to.
**These three points are not at all separate from one another, of course; when you emphasize a separation between the physical world and a spiritual world which is more true, it starts to seem like true knowledge must come from means other than those by which we gain knowledge about the physical world. And, similarly, it can easily follow that the creator of this physical world is not exactly the same as our spiritual Father.
***Note the lowercase “o” - this refers not to Orthodox Christianity, but to the whole lineage of mainstream Christianity, including Catholics, Orthodox, and most Protestants.
****The term is used in still another sense in the context of Chaos Magick and related traditions, in which it is used to refer to basically any altered state of consciousness. This is a nonstandard use of the term, but it does come up fairly often.