Sentence:  If a = 0, then x = ib is called an <WSD>imaginary number</WSD>. The message here is that we must introduce <WSD>imaginary numbers</WSD> in order to be able to solve quadratic equations in general. 
Meaning of the word marked with <WSD> tags:  A number of the form "bi", where "b" is any real number and "i" denotes the imaginary unit.