A payable-on-death account is a bank or credit union deposit account on which the owner has named one or more beneficiaries to receive the balance when the owner dies. It requires no lawyer, no trust document, and no change to how the account works day to day. The owner keeps complete control while alive, and at death the named beneficiaries claim the balance directly from the institution with a death certificate and identification, bypassing probate.
The FDIC's rules supply the definition and, unusually, the whole synonym set in one sentence. 12 CFR 330.10(a)(1) defines an informal revocable trust as "a trust under which a deposit passes directly to one or more beneficiaries upon the depositor's death without a written trust agreement, commonly referred to as a payable-on-death account, in-trust-for account, or Totten trust account." Three names, one arrangement, and the regulation says so itself.
The naming deserves a moment, because two of those names mislead. "Totten trust" is a historical label from an early-twentieth-century New York case, and it survives in the regulation and in older paperwork rather than in ordinary use. And although both that name and the FDIC's category use the word "trust", a payable-on-death account is not a trust in the estate-planning sense. There is no trust document, no trustee, no separate legal arrangement, and nobody holds anything for anybody's benefit while the owner is alive. It is a deposit account with an instruction attached to it.
It also is not the same instrument as a transfer-on-death registration, which is the equivalent mechanism on brokerage accounts and, in some states, on real estate. The idea is the same and the governing rules are not.