Is it possible to "merge" tags?

I'm not sure what word applies here.

Example:

Let' say I have a copy of Plato's Republic and I apply the tag "Greek Philosophy"
Let's say I have a book about Greek history, tagged "Greece, History"

Can I merge the tags so that when I search on Greece, it also pulls up books with the "Greek" tag?

I guess instead of "merge" I should have called "associating."