A description of a data structure that is independent of machine- oriented structures and encodings. [Source: RFC1208

Implementation-independent way of representing Application layer data or application or presentation protocol control information (PCI). Consists of a group of data types that describe a set of data structures that several applications can share.

Abstract syntax is a representation of data (typically either a message passing over a communications link or a computer program being compiled) which is independent of machine-oriented structures and encodings and also of the physical representation of the data (called "concrete syntax" in the case of compilation or "transfer syntax" in communications).