1. The technique uSed by layered protocolS in which a layer addS header information to the protocol data unit (PDU) from the layer above. AS an example, in Internet terminology, a packet would contain a header from the phySical layer, followed by a header from the network layer (IP), followed by a header from the tranSport layer (TCP), followed by the application protocol data. 2. The ability to provide uSerS with a well-defined interface to a Set of functionS in a way which hideS their internal workingS. In object-oriented programming, the technique of keeping together data StructureS and the methodS (procedureS) which act on them. (1998-09-07)