The decade beginning in '90 and ending in '99 of a century. / Temperatures from 90 to 99 degrees.

Alternative spelling of 90s

(physics) The radioactive isotope of strontium, ⁹⁰₃₈Sr, that is present in the fallout from nuclear explosions and, being chemically similar to calcium, is accumulated in the bones

A romanticized vision of the 1890s; compare with roaring 20s. / (derogatory) The 1990s, perceived as mediocre in comparison.

(physics) An isotope of yttrium, ⁹⁰₃₉Y, that is used in radiotherapy

Alternative form of 90s

(cellular automata) An elementary cellular automaton based on the exclusive-or function.

(aviation, military, US, historical) The original designation of the Lockheed XF-90, redesignated in 1948.

plural of XP-90

(US Army, US Air Force and US Navy, derogatory, slang) Newly-commissioned graduate of three-month Officer Candidate School (Army, Air Force), Midshipman School (Navy), or Direct Commissioning program.