The word Catholic means universal because that is the origin of the term. It's frοm the Greek term κατολικός , which means something like 'general, universal', and itself appears to be derived from καθ´ ὅλον, "according to the whole".

The Roman Catholic church terms itself the Catholic church because it has an institutional continuity stretching back to the early church and the term originated then as an understanding of the churches being a singular entity.