Why are normal subgroups called "Normal"? Who is credited with naming them, and why are they named such?
The following is a quote from http://jeff560.tripod.com where you can find some of the "Earliest Known Uses of Some of the Words of Mathematics":
NORMAL SUBGROUP. According to Kramer (p. 388), Galois used the adjective "invariant" referring to a normal subgroup.
According to The Genesis of the Abstract Group Concept (1984) by Hans Wussing, "The German Normalteiler (normal subgroup) goes back to Weber [H., Lehrbuch der Algebra, vol. 1, Braunschweig, 1895. p.511] and is possibly linked to Dedekind's term Teiler (divisor), which was employed in ideal theory" [Dirk Schlimm].
Normal subgroup is found in English in 1908 in An Introduction to the Theory of Groups of Finite Order by Harold Hilton: "Similarly, if every element of G transforms a subgroup H into itself, H is called a normal, self-conjugate, or invariant subgroup of G (or 'a subgroup normal in G')."
G. A. Miller writes in Historical Introduction to Mathematical Literature (1916), "In the newer subjects the tendency is especially strong to use different terms for the same concept. For instance, in the theory of groups the following seven terms have been used by various writers to denote a single concept: invariant subgroup, self-conjugate subgroup, normal divisor, monotypic subgroup, proper divisor, distinguished subgroup, autojug."
See here for the sources.