Absolute stereochemistry is not a chemical substance but a concept in chemistry referring to the precise three-dimensional arrangement of atoms in a chiral molecule, typically described using R/S or D/L notation. It plays a critical role in determining the biological activity, pharmacokinetics, and toxicity of pharmaceutical compounds. In drug development, knowing the absolute stereochemistry is essential because enantiomers can exhibit drastically different effects—one may be therapeutically active while the other is inactive or even harmful. It is also vital in asymmetric synthesis, where selective production of one enantiomer is required for high-purity intermediates in agrochemicals, fragrances, and fine chemicals. Analytical techniques such as X-ray crystallography, NMR, and circular dichroism are used to determine absolute configuration, ensuring correct structural assignment in research and quality control.