An enantiomer is a type of stereoisomer that exists as a pair of molecules which are mirror images of each other. These molecules cannot be superimposed, much like how left and right hands are mirror images but not identical. Enantiomers have the same molecular formula and connectivity of atoms, but their spatial arrangement differs, leading to distinct properties. Enantiomers are important in fields like pharmaceuticals, where one enantiomer of a drug may be effective while the other could be inactive or even harmful. This specificity arises because many biological systems, including enzymes and receptors, are chiral and interact differently with each enantiomer.