PDU - Protocol Data Unit

PDUs are relevant in relation to one of the first 4 layers of the OSI model as follows:

1. The Layer 1 PDU is the bit
2. The Layer 2 PDU is the frame
3. The Layer 3 PDU is the packet
4. The Layer 4 PDU is the segment (e.g. TCP segment)

(Layer 5 and above are referred to as data.)

Reference::

http://en.wikipedia.org/wiki/Protocol_data_unit