symmetric monoidal (∞,1)-category of spectra
While a homomorphism of magmas (including groups, rings, etc) must preserve multiplication, an antihomomorphism must instead reverse multiplication.
Let $A$ and $B$ be magmas, or more generally magma objects in any symmetric monoidal category $C$. (Examples include groups, which are magmas with extra properties; rings, which are magma objects in Ab with extra proprties; etc.)
An antihomomorphism from $A$ to $B$ is a function (or $C$-morphism) $f\colon A \to B$ such that:
Note that for magma objects in $C$, the left-hand side of this equation is a generalised element of $B$ whose source is ${|x|} \otimes {|y|}$ (where ${|x|}$ and ${|y|}$ are the sources of the generalised elements $x$ and $y$ and $\otimes$ is the tensor product in $C$), while the right-hand side is a generalised element of $B$ whose source is ${|y|} \otimes {|x|}$. Therefore, this definition only makes unambiguous sense because $C$ is symmetric monoidal, using the unique natural isomorphism ${|x|} \otimes {|y|} \cong {|y|} \otimes {|x|}$.
An antiautomorphism is an antihomomorphism whose underlying $C$-morphism is an automorphism.
In a $*$-algebra the $*$ operator is an antiautomorphism (in fact an anti-involution).