MDF stands for the main distribution facility, usually the biggest, but more certainly the most important, wiring closet. This is usually located at a central point in a star topology network that is then branched off into other wiring closets; routers, patch panels and hubs can usually be located here. An MDF must comply to the ANSI/TIA/EIA-569-A rules governing wiring closets and are usually located close to the POP (point of presence).
2002.11.19@11:24 spiregrain says re mdf: I've normally heard MDF=main distribution frame, where two-pair telephony connections are provided.