2.  An artificial entity created of astral light by magical intent or religious devotion, often in the form of a guardian creature (such as an Eagle or Lion) to protect a building or a spiritual group. The form can originate in mythology or be adapted from folklore and can be ritually charged with additional functions such as observing distant events, carrying out an act of revenge, or to serve as a focus of devotion to unite the group. 

3.  Invisible power that is the result of a group working ritually. This egregore may progressively become autonomous and independent from its source.

Consider that you are composed of two parts: a spiritual (invisible) part and a material (visible) part. You can apply the same concept to groups. Any group can be considered as a meeting of visible and invisible bodies. There is more to a group than the mere sum of its parts. A ritual group (Lodge, Coven, Chapter, etc.) gives birth throughout its existence and during each of its ritual practices, to a kind of independent psychological creature, which progressively develops its own unique character, thereby becoming more and more effective. This identity is larger than any individual participant and is commonly considered by initiates to be something called an Egregore or a "thought-form" (to use the Theosophical expression created by H.P. Blavatski). Over the years this archetype somehow becomes autonomous. This may be helpful or not, depending on whether the character of the group (and therefore the Egregore) is well balanced.