A deed of trust is the instrument by which a borrower pledges real property as security for a debt, transferring a limited ownership interest in the property to a trustee to hold until the debt is paid. Despite the word "deed" it does not transfer ownership to a buyer, and despite the word "trust" it is not an estate-planning trust: no one is managing assets for a beneficiary's benefit, and the "trust" here is a bare holding of a power of sale. It is the counterpart of a mortgage, used in place of one in many states, and a borrower signs it alongside the note that creates the debt.
California's Civil Code section 2924 treats the two side by side. Its opening sentence deems a transfer of rights in property, other than one made in trust, that is "made only as a security for the performance of another act", to be a mortgage, and the rest of the same subdivision then applies the identical notice-of-default and sale machinery to "any transfer in trust" where "a power of sale is conferred upon the mortgagee, trustee, or any other person". Two instruments, one procedure. Section 2920(b) then sweeps up whatever is left, providing that for the purposes of those same sections "mortgage" also means any security device conferring a power of sale over real property "other than a deed of trust", which section 2924 has already reached by name.