anorthositePronunciation: /əˈnɔːθəsʌɪt/Definition of anorthosite noun [mass noun] Geology a granular igneous rock composed largely of labradorite or another plagioclase. Origin: mid 19th century: from French anorthose 'plagioclase' + -ite1anorthosite in other Oxford dictionaries Definition of anorthosite in the US English dictionary