A deck transformation or cover automorphism is an automorphism of a covering space relative to the base space.

i.e. if $p:E\to X$ is a cover then a cover automorphism $f\in deck(p)=\{f|f\in Aut(E), p\circ f=p\}\subseteq Aut(E)$ is an automorphism of $E$ such that $p$ is invariant under composition with $f$.