What is PSBT? What does it stand for?
bitcoin-qt
has menu items "Load PSBT from file…" and "Load PSBT from clipboard…"; the dialog box says the file format is "Partially Signed Transaction".
It stands for "Partially Signed Bitcoin Transaction".
Bitcoin Core's documentation describes it as:
PSBT is an interchange format for Bitcoin transactions that are not fully signed yet, together with relevant metadata to help entities work towards signing it. It is intended to simplify workflows where multiple parties need to cooperate to produce a transaction. Examples include hardware wallets, multisig setups, and CoinJoin transactions.