ODU is DIRECTV's acronym for Outdoor Unit. It's a common enough term in the industry, so common that you don't tend to think about why it's ODU and not just OU which you sort of think it ought to be.
Specifically, an ODU consists of the following items, all assembled so that everything works:
- Mast and Mount: something to put it all on
- BAU (Back Adjustment Unit) Connects to the mast and lets you aim the dish)
- Reflector: The "dishy" part of the dish
- Arm: The part that sticks out from the reflector
- LNB (Low Noise (amplifier)/Block (downconverter): The part that receives the signals, out in front.
Put them all together and you have a working ODU. An ODU can also contain a multiswitch, which lets more than one receiver share the signals.