Entry 1 sense Webster, 1913 Alkalinity /ălˌ-kəl-ĭn'-ət-ē/ · Al·ka·lin·i·ty · IPA /ˌælkəlˈɪnətiː/ 01 n. The quality which constitutes an alkali; alkaline property. 1. The quality which constitutes an alkali; alkaline property.