Apart from industry hype it's easier to say what big data is not. To begin with it is not Hadoop (see preface to this series: What is Hadoop?). Nor is it simply having lots of data. And especially it is nothing to do with having lots of transactional data
Let's think about data growth for a moment. The first thing to note is that petabyte scale storage issues are not new: CERN had a distributed Objectivity-based database holding a petabyte back in the 90s, long before the large Hadron collider was much more than a dream in the eye of most physicists. All that's happened is that the commercial world is catching up on the scientific community. And, of course, it's all relative: what's big to me may be chickenfeed to you.