The term encompasses both the settlor of a testamentary trust (the testator) and the settlor of an express trust, (just as, for example, the term personal representative encompasses both the executor or the Court-appointed administrator of an estate).
Thus, the person who creates a trust within a will is, in law, all of a testator, a settlor and a donor.
The term is also used to describe the person who signs a power of attorney, and thereby gives his/her authority and powers to another, instead of the synonymous term, in that context, of principal.