How many views does it take for a video to go viral?

Whether a video is "viral" or not isn't really measured in hits. While there may be a correlation between the two, a viral video is really just a video that gets spread to a large number of people quickly. (Like a virus, get it?) Even by this standard there's no definite limit that makes a video count as viral, as people's definitions of a "large number" and "quickly" may be different.