PDU is short for "Protocol Data Unit". This basically means an amount of information delivered through a network layer.
Colloquial term is "packet" But that is not the same as PDUs from higher network layers may contain more data than a PDU from a lower layer may carry. In this case, the higher layer PDU is split into several PDUs from the lower layer. Furthermore, a PDU from a lower layer might contain multiple PDUs from higher network layers.