Part of the Magick in Theory & Practice series.
To "invoke" is literally to "call in" or to "speak within", in contrast with evocation, which is calling or speaking out. In magick, invocation is the process of bringing a desired god or similar entity within oneself.
Invocation as identification
The formula of invocation is ultimately one of identification; that is, to invoke a god successfully is to become that god (or equivalently, for that god to become you). Often this ultimate identification is preceded by several preparatory stages in which the god is addressed externally through descriptive salutations, pleas, and similar techniques. All such preparations are intended to arouse the magician to the point that successful identification becomes possible.
It is in this particular that invocation differs from other techniques for dealing with spirits. Rather than commanding, pleading, adoring, or otherwise interacting with the being as a separate entity, one becomes the entity and speaks one's will as that entity. If it is desired that the goddess Venus should bring one love, one does not implore Venus for assistance; one becomes Venus and states with absolute, calm assurance that love will come.
The Invocation of the Holy Guardian Angel
There is another, more esoteric type of invocation involved in the Great Work, that of the invocation of the Holy Guardian Angel. While this can be seen as a type of identification as well, it more closely follows the model of a marriage, and is expected to result in a bond which endures for life rather than the duration of a particular ceremony.
Other uses of invocation
It is also the case that invocations can be used for purely exploratory or sensual purposes, without a particular end desired. For example, an astral exploration of the sphere of Netzach, associated with Venus, might be preceded by an invocation of Venus to attune the magician to the appropriate symbols and energies.
Traditionally, rituals intended as invocations involve deosil (clockwise) motion, mirroring the direction of the Earth's rotation. This may be contrasted with rituals of banishing, which traditionally move widdershins (counterclockwise), opposing the Earth's rotation. Thus for example the Lesser Banishing Ritual of the Pentagram is technically closer to being an invocation than a true banishing; in this case, it is the archangels that are invoked as guards for the circle.