A straight line (trendline) that is drawn upward to the right across a chart along successive troughs (reaction lows). In other words, this line is drawn below the rising market troughs, i.e., under the successively higher reaction lows. A tentative up trendline would need two points to be drawn, whereas a third point is needed to confirm its validity.

The opposite of an up trendline is a down trendline.

For a graphic depiction, see: up trendline-figure.