a tree like structure, dynamically built up by the call of subtransactions until a bottom implementation level is encountered

A transaction begun within the scope of another transaction. These are also referred to as subtransactions.

a transaction that is enclosed within another transaction. The nested transaction behaves as if it were a single operation within the enclosing transaction.

With reference to a database transaction, a nested transaction occurs when a new transaction is started by an instruction that is already inside an existing transaction. The new nested transaction is said to be nested within the existing transaction, hence the term.