(physics) Any of two or more forms of an element where the atoms have the same number of protons, but a different number of neutrons within their nuclei. Thus, isotopes have the same atomic number but a different mass number.

(topology, transitive) To define or demonstrate an isotopy of (one map with another).

A line on a graph or chart, such as a weather map, along which all the points have the same temperature.

(meteorology) A line drawn on a map or chart connecting places of equal or constant pressure. / (thermodynamics) A set of points or conditions at constant pressure.

(chemistry) Any of a number of compounds that exhibit structural isomerism

(chemistry) Either of a pair of geometric isomers, one having the cis- and one having the trans- configuration.

(chemistry, dated, deprecated by IUPAC) either the cis- or trans- isomer of a compound that exhibits geometric isomerism

(chemistry) Any one of a number of compounds that exhibit optical isomerism.

(organic chemistry) An unsaturated hydrocarbon, C₅H₈, that is readily polymerized; natural rubber (caoutchouc) is cis-1,4-polyisoprene, and trans-1,4-polyisoprene is present in gutta-percha and balata; it is the structural basis for the terpenes.

(chemistry) any isomer that differs from another isomer only in the position of a substituent, or of a double or triple bond