Generally the "bottoming out" price of a stock in day trading. The stock can, but typically does not, dip below the "support level".
Example: If XYZ has support level of 45, and is at 46, it's a good indication to buy. It could drop lower, but likely won't drop below 45.
Note, the inverse of support is resistance.
Related terms: trendline, downtrend, and uptrend.