A self-explanatory term for a pharmaceutical agent in a specific chiral (enantiomeric) form. Because most biomolecules–e.g., proteins, sugars, etc. are found in only one of many chiral forms, different enantiomers of a chiral molecule will bind differently (or not at all) to a cognate target. Individual enantiomers may have markedly different effects.
The classic example of these differences is thalidomide, for which one enantiomer is effective for managing morning sickness, while another is highly teratogenic and causes phocomelia (flipper limbs), with the two enantiomers being converted into each other in vivo.* In the past, most drugs were racemic mixtures of a particular chemical or drug. Improved manufacturing processes have allowed the production of purified enantiomers, which bind more effectively and predictably to target proteins and receptors.
*Other examples include ethambutol–one enantiomer treats tuberculosis, the other causes blindness; and naproxen–one enantiomer treats arthritis pain, the other causes liver poisoning with no analgesic effect.