ACH operator is the short form of Automated Clearing House (ACH) operator, the central clearing facility that sits between banks in the ACH network, and that entry covers the role in detail.
In the United States there are two ACH operators: the Federal Reserve Banks, through their FedACH service, and The Clearing House, through its Electronic Payments Network. Each receives ACH files from originating banks, checks them, sorts the entries by receiving bank and delivers them, exchanging traffic with the other operator where the receiving bank is connected only to it. The operators also calculate each bank's net position and arrange settlement through accounts at the Federal Reserve, a step described in ACH operator settlement.
The two names are used interchangeably. The operator does not write the network rules, which come from Nacha, and it does not deal with businesses or consumers directly; those relationships run through the ODFI and the RDFI.