I've tried to come up with of a good definition and/or description of Content Engineering, but the Wikipedia entry for Content Engineering already does a great job:
Content Engineering is a term applied to an engineering speciality dealing with the issues around the use of content in computer-facilitated environments. Content production, content management, content modelling, content conversion, and content use and repurposing are all areas involving this speciality. It is not a speciality with wide industry recognition and is often performed on an ad hoc basis by members of software development or content production staff, but is beginning to be recognized as a necessary function in any complex content-centric project involving both content production as well as software system development.
Content engineering tends to bridge the gap between groups involved in the production of content (Publishing and Editorial staff, Marketing, Sales, HR) and more technologically-oriented departments (such as Software Development, or IT) that put this content to use in web or other software-based environments, and requires an understanding of the issues and processes of both sides.
Typically, Content Engineering involves extensive use of XML technologies, XML being the most widespread language for representing structured content. Content Management Systems are often a key technology used in this practice though frequently Content Engineering fills the gap where no formal CMS has been put into place.
Source: Wikipedia (January 2014)
Another very interesting source of information is a presentation by Joe Gollner of Stilo e-Publishing solutions. In 60+ sheets he presents the core concepts underlying Content Engineering.