A Definition of Big Data
By Forresters Boris Evelson and Brian Hopkins:
Big data: techniques and technologies that make handling data at extreme scale economical.
And the explanation of the 4Vs: volume, velocity, variability, variety:

Original title and link: A Definition of Big Data (©myNoSQL)
via: http://blogs.forrester.com/brian_hopkins/11-08-29-big_data_brewer_and_a_couple_of_webinars