Pub. 946, How to Depreciate Property (2020), IRS:

The one who acts on behalf of another as a guardian, trustee, executor, administrator, receiver, or conservator.

Medicaid Liens and Recoveries (02 OMM/ADM-3), April 17, 2002, has a narrow definition of fiduciary that applies only to Medicaid estate recovery in New York:

an executor named in the will of a deceased person (decedent), an administrator for a person who died without a will, or any other executor or administrator, who has been issued the appropriate letters by the Surrogate’s Court and is obligated to settle and distribute the decedent’s estate; or a voluntary administrator of the personal property of a small estate.

Significantly, this definition does not include a trustee. This definition reflects the rule that New York allows Medicaid recovery only from the probate or intestate estate. See estate.