Michael Stonebraker's answer is that big data can mean big volume (and small analytics,) big volume and big analytics, big velocity, and/or big variety. The first three aspects fit the general usage on HN, while the fourth is his own addition, I think. He's writing a blog post on each one. The first post explains what he means by each aspect:

Intro plus the "big volume, small analytics" case: http://cacm.acm.org/blogs/blog-cacm/155468-what-does-big-dat...

"Big volume, big analytics": http://cacm.acm.org/blogs/blog-cacm/156102-what-does-big-dat...

"Big velocity": http://cacm.acm.org/blogs/blog-cacm/157589-what-does-big-dat...

"Big variety": yet to come