An ACH debit is the formal Nacha term for an ACH pull payment, in which the business or person being paid instructs its bank to collect money from the payer's account through the Automated Clearing House network. That entry covers the process in full.
ACH debits are used for utility bills, insurance premiums, loan and mortgage repayments, subscriptions and many business-to-business collections. Because the payee starts the payment, it must first hold a valid ACH authorization from the account holder. If a debit fails or is disputed, the payer's bank sends it back as an ACH return, for example for insufficient funds or because the payer says it was not authorized.
"ACH debit" and "ACH pull" describe the same payment. The word "debit" reflects how the entry is posted to the payer's account, while "pull" describes who starts it. The reverse type is the ACH credit, or push. Businesses that want to stop unwanted pulls can ask their bank for an ACH debit block.