Protocol data unit

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.

Source: from Federal Standard 1037C