ADSL Termination Unit-Central Office. A piece of hardware located in a DSLAM (Digital Subscriber Line ATM Multiplexor) at your local CO. It lies between an ADSL user's line and the switches in the PSTN.

This piece of hardware is responsible for taking the line that goes from your home and connecting it to the DSL Provider's network. What this means to a user is if it gets broken then you're in bad shape because you can't travel any farther then that into the network.