Orosirian

EtymologyName formalityFormalUsage informationCelestial bodyEarthRegional usageGlobal (ICS)Time scale(s) usedICS Time ScaleDefinitionChronological unitPeriodStratigraphic unitSystemTime span formalityFormalLower boundary definitionDefined chronometricallyLower GSSA ratified1990Upper boundary definitionDefined chronometricallyUpper GSSA ratified1990

The Orosirian (; Ancient Greek: ὀροσειρά, romanized: oroseirá, meaning "mountain range") is the third geologic period in the Paleoproterozoic Era. It spans 250 million years and lasted from 2050 to 1800 million years ago (Ma), following the Rhyacian Period and preceding the Statherian Period. Instead of being based on stratigraphy, these dates are defined chronometrically.