Entry 1 sense Webster, 1913 Alumina /əl-o͞om'-ən-ə/ · A·lu·mi·na · IPA /əˈl(j)uːmɪnə/ 01 n. One of the earths, consisting of two parts of aluminium and three of oxygen, Al2O3. 1. One of the earths, consisting of two parts of aluminium and three of oxygen, Al2O3.(Chem.)