A Molecule that has the same Chemical formula as another molecule, but differs in the orientation of its atoms in space. This can happen by swapping some Moietyes by their mirror images

If the molecule is just the mirror image of the other molecule, it is called an Enantiomer, and if it isn't, it is a Diasteromers. If two steroisomers are actually the same (can be made to coincide after a rotation) they are known as meso compound