A group involution is an element of order 2 in a group (i.e., an element 
of a group such that 
, where 
is the identity element).
Group Involution
See also
Group, Identity ElementExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Group Involution." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/GroupInvolution.html