Isotope, one of multiple varieties of the same element of different atomic masses

Isomer, one of multiple molecules with the same formula

a contour line of equal or constant pressure in meteorology

a heat pipe

Nuclear isomer, a nuclide in a neutral state (i.e., containing more energy, but with no change in number of protons and neutrons)

two nuclides with the same mass number in nuclear physics

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Isobaric process

An isosurface is a three-dimensional analog of an isocontour. It is a surface that represents points of a constant value (e.g. pressure, temperature, velocity, density) within a volume of space; in other words, it is a level set of a continuous function...

Isotone, one of multiple nuclides with equal numbers of neutrons