There are many negatives weighing on the market right now, but Art Cashin of UBS Financial Services says of all the headwinds, the price of crude oil is the most serious.
"It has returned to the downside," Cashin said Wednesday on CNBC's "Power Lunch," and it's the weakness in oil that has spooked pros on the NYSE floor.
Although the threat of as well as unrest in the and Russia are also problematic for stocks, Cashin said as negative catalysts they're not as powerful as oil.
And he added the outlook is reflected in the price action, earlier in the session. "When the market turned at 9:45 a.m., if you look at a chart of oil, that's when oil went positive," Cashin said.
In other words, the market and oil are almost trading in lockstep. And after 12 p.m. oil turned lower and the market tumbled. Then, late day as oil pared losses, so did the .
Looking ahead to Thursday, Cashin added if oil drops below $80 in a meaningful way, the stock market could go from bad to worse. "What happens in the energy markets will be very important," Cashin added in an appearance on "Closing Bell."
Read MoreCorrection watch: Here are the official levels
"Below $80, oil will raise concerns about real deflation," Cashin said. " are already beginning to see literal deflation." If WTI slips below $80, Cashin says pros will fear deflation is wider spread that previously believed.
And, taken a step further, Cashin says if deflation becomes a global phenomenon, traders will run for cover. "It's a worldwide threat and it seems to be coming home to roost."