Unable to throw out a definition that satisfied me, I turned to the browser.
Trying to find the Latin root, the closest I got was "egregius", meaning distinguished or illustrious. Cute, but doesn't sound related.
This following is from Gaetan Delaforgem from a Gnosis article, "The Templar Tradition: yesterday and today":
..."An egregore is a kind of group mind which is created when people consciously come together for a common purpose. Whenever people gather together to do something and egregore is formed, but unless an attempt is made to maintain it deliberately it will dissipate rather quickly. However if the people wish to maintain it and know the techniques of how to do so, the egregore will continue to grow in strength and can last for centuries.
An egregore has the characteristic of having an effectiveness greater than the mere sum of its individual members. It continuously interacts with its members, influencing them and being influenced by them. The interaction works positively by stimulating and assisting its members but only as long as they behave and act in line with its original aim. It will stimulate both individually and collectively all those faculties in the group which will permit the realization of the objectives of its original program. If this process is continued a long time the egregore will take on a kind of life of its own, and can become so strong that even if all its members should die, it would continue to exist on the inner dimensions and can be contacted even centuries later by a group of people prepared to live the lives of the original founders, particularly if they are willing to provide the initial input of energy to get it going again.
If the egregore is concerned with spiritual or esoteric activities its influence will be even greater. People who discover the keys can tap in on a powerful egregore representing, for example, a spiritual or esoteric tradition, will, if they follow the line described above by activating and maintaining such an egregore, obtain access to the abilities, knowledge, and drive of all that has been accumulated in that egregore since its beginnings. Agroup ororder which manages to do this can, with a clear conscience, claim to be an authentic order of the tradition represented by that egregore. In my view this is the only yardstick by which a genuine Templar order should be measured."
Aside from the shite about orders, not too bad.
This bit from Frater UD on Fenwick's page seems more of a response to
this type of thought than a definition, but interesting:
In this work from Temple Babel on Tzi's site, the word is used basically
interchangeably with demon:
In Simon's Necronomicon, I recall that the Watcher was said to be of "the genus Egregorus" or somesuch. I have also heard the term used alongside "Angel".
Interesting bits from the Tiamat list on Independent Thoughtforms:
From EE Rehmus' Magician's Dictionary:
Watchers, thought-form entities created by visualization, ritual and such. They come in collective groups. They are somewhat like angels, except that they are relatively mindless and quite willing to follow orders. Some ufos may be egregores.
A second significant reason for insisting on this point is the subject of the egregores. What is an egregore? It is the psychic and astral entity of a group. All members of a group, a family, a club, a political party, a religion or even a country, are psychically included in the egregore of the organization to which they belong. Of course, each of us belongs to several egregores at once. Therefore, each individual who is involved in a group receives the influences of the egregores, that is the astral counterpart of the group, in his psyche. This process is unconscious. The resulting drawbacks are, first, some perturbating psychic influences in the majority of cases, and second, a restriction of inner freedom. It is impossible to free oneself from certain egregores, for example the egregores of the country you live in. However, we should free ourselves from all egregores which are not essential. An egregore actually grows by drawing support from the members which constitute it who, in turn, through their repeated actions vivify it, somehow helping it to maintain its power. For a beginner, this is where the danger lies, all the more because of the tendency of man to seek protection, the price of which is often a loss of freedom. We should emphasize here that the association the Philosophers of Nature does not perform any group ritual in order to reduce the influence of its egregore to a minimum.
That's it for now. I'll keep digging.