Term for same root word but words with different meaning

The word to describe this situation (about words) is cognate. 'Skirt' and 'shirt' are cognate because they have a word in common etymologically.

There are actually lots of these. They are called doublets.

My favorite example, which is not mentioned on that Wikipedia page, is the following:

Genre, gender, generic, and genus all come from the Latin genus meaning "kind/class" (although the word actually goes all the way back to Proto-Indo-European *gen-/*gon-/*gn- meaning "produce, beget, be born").

Just to point out, doublets are cognates within a single language.

My favorite pair is galaxy and lactose, which come from the Greek word for milk, gala. The genitive is galaktos. Apparently, Milky Way wasn't a weird choice of name at all...

Cognates with different meanings from different languages/dialects are known as False Friends.

My favourite pair is "canon" (=rule) and "cannon" (=weapon). Both from the same Greek word (kanna) meaning a reed, probably from a semitic root. I think that is an awesome stretch of meaning.