(mathematics) Synonym of pentacontagon: A 50-sided polygon.

(sports) The act of spinning 540 degrees in the air, making 1½ turns.

(Canada, US, fiction) Pertaining to a fake telephone number. The local exchange and area code number 555 was reserved in North American dialing plan for fictional numbers, prior to the 21st century.

(singular only) The list published annually by Fortune magazine of the 500 United States companies with the highest revenue. / (plural only) Collectively, the members of that list.

(physics) A minor stable isotope of titanium, ⁵⁰₂₂Ti, having twenty-two protons and twenty-eight neutrons; it amounts to about 5% of the element in nature

(physics) The only stable isotope of vanadium, ⁵¹₂₃V, having twenty-three protons and twenty-eight neutrons

(informal, US, military, aviation, research, espionage, ufology) A secret military facility, an airport, an airbase in Nevada Test and Training Range, Nevada, United States; A military base in Nevada, where advanced military aircraft are developed; foreign military aircraft are evaluated; and rumored to be where the US government hides extra-planetary alien visitors.

The decade of the 1950s. / Temperatures from 50 to 59 degrees.

(physics) The naturally-occurring radioactive isotope of vanadium, ⁵⁰₂₃V, having twenty-three protons and twenty-seven neutrons; its half-life is 1.5 x 10¹⁷ years

(athletics) A long-distance running race over 5000 metres.