What do you call having the ability to act or change without constraint?
Freedom, generally, is having the ability to act or change without constraint. Something is "free" if it can change easily and is not considered in its present state. At a subjective level, freedom is is often associated with one's ability to meet their needs, wants, and aspirations. In philosophy and religion, it is associated with having 'free will' and being without undue or unjust constraints, or enslavement, and is an idea closely tied with the concept of liberty.
A person has freedom to do things that will not, in theory or in practice, be prevented by other forces. Outside of the human realm, freedom generally does not have this political or psychological dimension. A rusty lock might be oiled so that the key has the freedom to turn, undergrowth may be hacked away to give a newly planted sapling freedom to grow.
Freedom does not mean doing whatever you like. True freedom implies that a person is able to do things that will benefit himself and others in the community. In some circumstances, particularly when discussion is limited to political freedoms, the terms "freedom" and "liberty" tend to be used interchangeably. Elsewhere, however, subtle distinctions between freedom and liberty have been noted.