Gladwell based his research in part on the work of two great social scientists.
First, Stanley Milgram's 1967 study of social networks. A seminal work spawning many books and articles including the recently published Linked: The New Science of Networks by Albert-Lasio Barabasi and Six Degrees of Separation by Duncan Watts.
The phrase was coined by Morton Grodzins, who studied integrating neighborhoods in the early 60's. He discovered that most white families would remain in the neighborhood so long as the comparative number of black families remained very small. But, at a certain point, when "one too many" black families arrived, all would move out in mass. He called that moment "the tipping point."
Therefore, the tipping point is that dramatic moment when something unique becomes common.
To put across his point, Gladwell explains how and why a simple disease becomes an epidemic only if it meets three criteria: The Law of the Few, The stickiness factor, and the power of context.
Gladwell expands his thesis into the business arena with the Magic Number 150 -- "the maximum number of individuals with whom anyone can have a genuinely social relationship," and a wonderfully simple bell curve graphic that demonstrates how a new idea, initiative, product, publication or website must first reach "Innovators" who are connected to "Early Adopters" who in turn energize the "Early Majority" that initiate the tipping point.