My simple definition-
The process of moving an organization and its people to a different and better state.
A good change agent does this in a way that links the context of work to the bigger picture, enlivens the culture of the organization, makes the right lists for the right people and does so with an empathetic and business approach.
A really good change agent (here I am thinking exclusively external although with the right leverage and status it could be possible for an internal change agent) does all of the above while also improving operational efficiency, developing employees-especially future leaders and layering in a replicable process for the next corporate initiative.
Any large engagement, and I would say a lot of the well budgeted smaller ones, needs someone to fill these responsibilities. Finding people to do the implementing/project work (which is where you might have thought my answer was going to go) is the easy part.