Sentence:  We saw in the previous section that <WSD>monotone functions</WSD> have some nice properties. For example, a <WSD>monotone function</WSD> is continuous except possibly on a countable set. They are also closed under scalar multiplication, and the sum of two increasing functions is increasing. (Exercise (4) on page 103). However, the difference and product of two <WSD>monotone functions</WSD> are not necessarily monotone (Exercise (5) on page 103). 
Meaning of the word marked with <WSD> tags:  A function "f" : "X"→'''R''' (where "X" is a subset of '''R''', possibly a discrete set) that either never decreases or never increases as its independent variable increases; that is, either "x" ≤ "y" implies "f"("x") ≤ "f"("y") or "x" ≤ "y" implies "f"("y") ≤ "f"("x").