Protocol data unit (PDU) is a generic term to describe the length of a single piece of data transmission at any layer. The PDU includes both the header for its layer plus the payload coming from higher layers.
For OSI layer 1, the PDU is the bit.
For OSI layer 2, the PDU is the frame.
For OSI layer 3, the PDU is the packet.
For OSI layer 4, the PDU is the segment.