Elinvar is a nickel steel alloy with a modulus of elasticity which does not change much with temperature changes. The name is a contraction of the French Elasticité invariable. It was invented around the 1920s by Charles Édouard Guillaume, a Swiss physicist who also invented Invar, another alloy of nickel and iron, which has very low thermal expansion. Guillaume won the 1920 Nobel Prize in Physics for these discoveries, which indicates how important these alloys were for scientific instruments.

The largest use of Elinvar was in balance springs for mechanical watches and chronometers. A major cause of inaccuracy in watches and clocks was that ordinary steels used in springs lost elasticity slightly as the temperature increased, so the balance wheel would oscillate more slowly back and forth, and the clock would lose time. Chronometers and precision watches required complex temperature-compensated balance wheels for accurate timekeeping. Springs made of Elinvar, and other low temperature coefficient alloys such as Nivarox that followed, were not affected by temperature, so they made the temperature-compensated balance wheel obsolete.

Elinvar consists of 59% iron, 36% nickel, and 5% chromium. It is almost nonmagnetic and corrosion resistant.