(organic chemistry) Any of four isomeric univalent hydrocarbon radicals, C₄H₉, formally derived from butane by the loss of a hydrogen atom. / A synthetic rubber made by the polymerization of isobut...

A main road or highway, especially in ancient Rome. (Mainly used in set phrases, below.) / (electronics) A small hole in a printed circuit board filled with metal which connects two or more layers.

(malacology) A shell having the form of a coiled spiral (in which the whorls impress each other) like a nautilus.

(grammar) The use of the to infinitive form of a verb in answer to the implied question why?.