Account control agreement is the shorter, everyday name for a bank account control agreement, and that entry covers the subject in full: the parties, the "springing" and "blocked" forms and how control is exercised in a default. This page gives a brief definition and explains why the two names exist.
An account control agreement is a contract between a borrower, its lender and the bank that holds the borrower's account. The bank agrees that it will follow the lender's instructions about the money in the account, usually only once the lender says the borrower has defaulted. In the United States this matters because, under Article 9 of the Uniform Commercial Code, a lender perfects a security interest in a deposit account by "control", and a signed agreement of this kind is the standard way to obtain it. The account itself is treated as collateral for the secured loan.
Practitioners often shorten the name to "ACA", and when the account is a deposit account it is also called a deposit account control agreement, or DACA. The word "bank" is dropped in daily use because the parties already know a bank is involved. Similar arrangements exist for securities accounts held with a custodian, where the agreement is sometimes called a securities account control agreement.