Essay:Will
Do what thou wilt shall be the whole of the law
love is the law, love under will.
- Aleister Crowley
Any discussion of Will in a magical context has to start here. Whatever "Will" meant before these words were commonly used has been permanently obscured by this phrase and it's associated texts.
Will in Thelema
In Thelema Will refers, roughly and approximately, to What You Must Be Doing With Your Energy. This is intensely paradoxical: Folk Thelema often borrows heavily from hedonistic or Satanic ideas about self definition - to step outside of social norms and indulge yourself. The idea that Will is Duty is largely absent from that culture, although clearly the intention behind this phrase was not "Do As You Like" and Crowley himself says as much on many occasions.
Up until Knowledge And Conversation of the Holy Guardian Angel your Will - your Dharma or Duty - is to reach that grade/state of being/experience. All activities which contribute to reaching that degree are known as Doing One's Will. At this Grade, the Angel becomes manifest and makes its will known to the magician, and its will becomes the True Will of the Magician.
Usually the Angel is said to have a purpose on earth: "Invent Solid Rocket Fuel" for example. Before the True Will is consciously known, a human being may be unconsciously moving towards their Will - acting unconsciously towards a goal which existed before they were born.
Note that this schema leaves very little room for Free Will. This is clearly not widely practiced among Thelemites of the modern age who seem to often confuse reckless self indulgence and antinomian behavior with Doing Their Will. In a limited sense, that kind of rule breaking may be part of the deconditioning process of the lower grades which enables the True Will to be discovered. Other than that, however, it has no place in this schema, Aleister Crowley's personal practices not withstanding.
Now, all Thelema aside, what is Will?
Will is a kind of internal muscle - we can feel when we exert it, tell when somebody has a strong one or is practiced in using it, but it is not clear where this muscle resides. In the Indian schema, Will is said to reside most visibly at the Third Chakra which is responsible for the sense of self and individual self-identity. Will exists, apparently, to compel entities or aspects of the self which do not wish to comply with the consious intention of the being to do so: to Will oneself to stop smoking, for example, or to will somebody else into doing the damn dishes.
Esoterically, Will is often associated with Fire, with the Wand, and with the concept of "First Cause," the idea being that the Divine in some sense "wills" the creation into existence, or sets the stars in motion with will.