It looks like it’s Warren Buffett’s turn to say, “I told you so.” Late last year, the Berkshire Hathaway CEO was busy selling stocks when the S&P 500 logged more than 50 record closes, leading many market observers to scratch their heads.

Such a precipitous increase in the value of U.S. equities has stirred up the usual cacophony of equity-bears screaming “bubble!” “Bubble” is a popular term, but, as it turns out, is rather ill-defined and fuzzy.

Warren Buffett is commonly regarded as the one of the greatest investors in American history. That's because Berkshire Hathaway ( BRK.A 0.56%) ( BRK.B 0.53%) stock has returned 19.9% annually since he took control six decades ago. Comparatively, the S&P 500 ( ^GSPC 1.12%) has returned 10.4% annually during the same period.