In telecommunications, the term protocol data unit (PDU) has the following meanings:

1. Information that is delivered as a unit among peer entities of a network and that may contain control information, address information, or data.
2. In layered systems, a unit of data that is specified in a protocol of a given layer and that consists of protocol-control information of the given layer and possibly user data of that layer. For example: Bridge PDU or iSCSI PDU

Source: from Federal Standard 1037C

It is sometimes used as a synonym for the more common term packet.

In telecommunications, the term packet data unit (PDU) has the following meaning:

1. The units that are transported in a LIN (Local Interconnect Network) diagnostic frame are called PDU (Packet Data Unit). A PDU used for node configuration is a complete message.

Source: LIN Diag and Config Spec Revision

Data transmission