The bull market in stocks, the longest on record since World War II, is so strong that even a 20 percent drop would not derail the long-term viability of the rally, Wall Street's Jeff Saut told CNBC on Tuesday.

By the standard definition, a bear market starts when the falls 20 percent from its bull market high. On the flip side, a bull market begins once the index rises 20 percent from its bear market low.

But Saut, chief investment strategist at Raymond James, thinks the letter of the law should not apply to the current market environment.

"I don't buy it," he argued in a "Squawk Box" interview. If a 20 percent decline were to happen, "it may be a tactical bear market. But it doesn't end a secular bull," defined as a market driven by forces in place for years.

Last week, the bull market that began on March 9, 2009 eclipsed the one that ran from Oct. 11, 1990 until the dot-com bubble burst on March 24, 2000. The S&P 500 has gained more than 300 percent since its low nine years ago.