1. Prefix designating a benzene compound in which two substituents are in the 1,4 positions, i.e. directly opposite each other, on the benzene ring. The abbreviation p- is used; for example, p-xylene is 1,4-dimethylbenzene. Compare ortho-; meta-.

2. Prefix denoting the form of diatomic molecules in which the nuclei have opposite spins, e.g. parahydrogen (see hydrogen). Compare ortho-.