Wikipedia, for example, gives this almost unintelligible description, and this is only the tip of the iceberg: there are almost as many definitions of NLP as there are practitioners...
...and so, not to be outdone, Kanaloa offers its very own contribution to the floating ice mountain:
NLP is a set of attitudes, beliefs and techniques collected around a theme of understanding how individuals think and communicate with themselves and others... ...and how this can be usefully applied.
A key feature of NLP is that it sits in the cusp between subjectivity and objectivity (rather like Pirsig's* famously undefined 'Metaphysics of Quality'). It is therefore not too surprising that there is a definition difficulty!
Conversely, there is more consensus over the building blocks used in NLP. This Glossary describes the key definitions and techniques used by most practitioners.
* R M Pirsig: 'Zen and the Art of Motor Cycle Maintenance' and 'Lila'