Language is compression of a sort. A dictionary is a decompressor. You look up one word, and you may get a paragraph about its meaning. An encyclopedia can be thought of as roughly the same with more detail for some nouns.

It makes a lot of sense why dictionary-based compression is named the way it is. A shorter symbol is used to store information that would take more symbols in the uncompressed corpus, if the shorter symbol hadn't been assigned to represent it. That's in a way just what an actual dictionary on your English professor's shelf does. The big difference is your compressor is coining new short symbols all the time.