The longest-running bull market of all time is on thin ice as the major indices sit in correction territory. The , Dow and Nasdaq have all plunged more than 10 percent from their respective highs as trade skirmishes and the Fed have kept investors on edge.

Four experts weigh in on whether this historic bull run is a thing of the past:

· DoubleLine Capital founder Jeffrey Gundlach thinks that the writing is on the wall, saying, "I'm pretty sure this is a bear market." A bear market is generally defined as a 20 percent move from peak to trough. However, Gundlach calls that definition "arbitrary" and says a number of other factors in the current market lead him to believe it's already here. "How you lead into it, how it develops, and how the sentiment changes. I think we've had pretty much all of the variables that characterize a bear market."

· Ryan Detrick of LPL Financial Research say that while this is one of the worst starts to December in the history, he is holding out hope that we could see a pop before the year's end. "We still think there's a chance that Santa Claus can come in the next two weeks. Historically speaking, these last two weeks of December are strong." For those wondering if a recession might be on the cards in 2019, the senior market strategist had this to say: "You don't see recessions when you see global earnings still growing."

· Bleakley Advisory Group CIO Peter Boockvar says there are indications that we may have seen a intermediary stop to the selling. "[Monday] when we sold off in utilities, REITs, and consumer non-durables, those were the areas of safety that people were hiding out in…That kind of selling means you probably get to a short-term bottom, but I emphasize short-term."

· Fundstrat's Tom Lee thinks that we have to pump the breaks on talk of a bear market. "To me, the word 'bear market' is a decline that precedes an economic downturn, and I think the market is pricing in a downturn that is not even as close as people think," says Lee. "I just think people have looked at the weakness in the rest of the world and think it affects the U.S. I don't think the bull market is over and I don't think the bull market's broken." Instead, Lee offers more good news: the longest bull market in history could have a long time left to run. "To us, it's just a transition year. We've been calling it, sort of, a mid-life. The bull market is reaching its mid-life."