What does the greek word euangelion mean?

It's a Greek word (ευαγγέλιον, pronounced evv-ann-ghell-ee-on).

It is made up of eu-(meaning good) and -angelia (meaning news), so basically it means good news.

Today it is used exclusively to refer to the four gospels of the New Testament (because to Christians they contain the good news that the Messiah has arrived).

The four evangelists, of course, are quite literally the bearers of good news.