A term with various uses.
The mapping $x +iy \mapsto x - iy$ of the complex numbers, which a field automorphism of order 2, fixing the subfield of real numbers. Analogous involutory automorphisms and anti-automorphisms of fields and skew fields such as the quaternions.
Conjugation. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Conjugation&oldid=35145