is a subnormal subgroup of 
if there is a "normal series" (in the sense of Jordan-Hölder)
from 
to 
.
Subnormal Subgroup
See also
Jordan-Hölder TheoremExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Subnormal Subgroup." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/SubnormalSubgroup.html