means One of two or more forms of the same chemical element that have the same number of protons but different numbers of neutrons, giving them different atomic masses.

from From Greek isos 'equal' plus topos 'place' — literally 'same place,' because isotopes sit in the very same spot on the periodic table despite their differing weights. The term was coined in the early 20th century, reportedly suggested by the writer and physician Margaret Todd to chemist Frederick Soddy, who needed a word for elements that were chemically identical yet physically distinct.