| context | $\langle\!\langle M,* \rangle\!\rangle$ … magma |
| definiendum | $\langle\!\langle {\mathcal P}M,\star \rangle\!\rangle$ |
| definition | $\star\in$ binary operation on ${\mathcal P}M$ |
| definition | $S\star T:=\{x*y\ |\ x\in S, y\in T\}$ |
When talking about left cosets etc., people write $xS$ for $\{x\}S$.
Wikipedia: Magma