/sci/ - Science & Math

What is a better way to word a "range interval" for a given function that more informatively denotes what it is? All I'm trying to do is describe the (dy) differential for a standard function of one variable, but I can't think of the most efficient way to word it. My first instinct was to call it the "difference in height" between the point on a line tangential to the given curve and another point along that line an arbitrary distance (dx) away" but to describe it as "height" completely misses the point. It's an interval of solutions or a solution set but even describing them as "solutions" implies that there was a problem that needed solving, which again is not relevant. How can I describe this interval of solutions in a way that doesn't miss the point? My mind is blank. Pic somewhat related.