A set $M$ endowed with an everywhere defined binary operation $m : M \times M \rightarrow M$ on it. No conditions are imposed. In particular, a magma need not be commutative or associative. Of particular importance is the free magma on an alphabet (set) $X$. A mapping $f : N \rightarrow M$ of one magma into another is a morphism of magmas if $f(m_N(a,b)) = m_M(f(a),f(b))$ for all $a,b \in N$, i.e., if it respects the binary operations.
Magma. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Magma&oldid=12223