Most times when someone asks you to define a word, they expect you to give them a string of other words that amount to the same meaning. But that's clearly not the only way to define things. Some terms have to be defined by reference rather than reduction. What is red?
Did you answer, "the range of visible light ranging in wavelength from 780 to 622 nanometers"? Because that's wrong. Not only does that definition fail to answer the question for someone who's never seen red or doesn't know the science behind it, but that's not what defines red. There could be an alien species with brains that perceive that range of light waves as blue. Put another way, that's what red happens to be for us, but not what defines it.
So what really defines red? This. To explain it, you have to point someone to an example and say, "that is red". This is called defining by reference. It's important because some very important words that are not things everyone already knows have to be defined this way. For example, Fuiki (I explain why defining this by reduction doesn't really work in the end of this article).