Defining By Reference

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 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:

https://www.google.com/search?q=red

To explain it, you have to point someone to an example and say, "*that* is red". This is called defining by reference.