A covering to which every other covering is subordinate.
A covering $p:X\rightarrow Y$ of a space $Y$ is subordinate to a covering $p':X'\rightarrow Y$ if there is a covering $f:X'\rightarrow X$ such that $p'=pf$.
Universal covering. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Universal_covering&oldid=11638