Sentence:  In what follows, we will use the <WSD>"mean value theorem</WSD>", another one of Lagrange's many contributions to numerical analysis. 
Meaning of the word marked with <WSD> tags:  The theorem that for any real-valued function that is differentiable on an interval, there is a point in that interval where the derivative of the curve equals the slope of the straight line between the graphed function values at the interval's end points.