Is there one?
All the definitions I can find describe the size, complexity / variety or velocity of the data.
Wikipedia's definition is the only one I've found with an actual number
Big data sizes are a constantly moving target, as of 2012 ranging from a few dozen terabytes to many petabytes of data in a single data set.
However, this seemingly contradicts the MIKE2.0 definition, referenced in the next paragraph, which indicates that "big" data can be small and that 100,000 sensors on an aircraft creating only 3GB of data could be considered big.
IBM despite saying that:
Big data is more simply than a matter of size.
have emphasised size in their definition.
O'Reilly has stressed
"volume, velocity and variety" as well. Though explained well, and in more depth, the definition seems to be a re-hash of the others - or vice-versa of course.
I think that a Computer Weekly article title sums up a number of articles fairly well "What is big data and how can it be used to gain competitive advantage".
But ZDNet wins with the following from 2012:
“Big Data” is a catch phrase that has been bubbling up from the high performance computing niche of the IT market... If one sits through the presentations from ten suppliers of technology, fifteen or so different definitions are likely to come forward. Each definition, of course, tends to support the need for that supplier’s products and services. Imagine that.
Basically "big data" is "big" in some way shape or form.
What is "big"? Is it quantifiable at the current time?
If "big" is unquantifiable is there a definition that does not rely solely on generalities?