The NBER definition of a recession is a significant decline in economic activity that lasts a few months or more. The most common layman's definition is negative GDP growth for two consecutive months. The thing is, recessions are really big movements within the economy, they take months to really get going. Also, there are all sorts of leading indicators that point to recessions, such as reduced job growth and increasing layoffs that result from corporate analysts that foresee recessions and want to get ahead of them by shedding labor costs early. Inflationary shocks can create recessions, as well as government actions that increase unemployment and thus reduce the consumer spending that is the backbone of the US economy.

If you see a small oddly-shaped mole you don't ignore it until it's become full-blown metastasized melanoma, and right now Trump is that mole on the US economy. He thinks he's bigger than the economy and that he can just wrestle it around to benefit himself, and the end result will, and is, a recession/depression that almost certainly will be worse than the Bush Great Recession and the Great Depression, combined. He's a monkey at the controls of a jumbo jet pushing buttons and twisting knobs because it makes the pretty lights go on and off.