A device (hard- and/or software) that decodes a MPEG2 Elementary Stream and prepares data for (e.g. audiovisual) presentation.
Since the decoding algorithms are usually quite complex (and thus implementation is error-prone) vendors are moving in modern STB and DVD player chipsets from fixed-function silicon to Firmware-driven designs which allow late hotfixes.
Very modern approaches even implement the entire decoding process in software on the host processor.
See also VDR Software Decoder Plugin for a further distinction of hardware and software decoding.