Entry 1 sense Webster, 1913 Insubstantial /ĭnˌ-səb-stăn'-shəl/ · In·sub·stan·tial · IPA /ˌɪnsəbstˈænʃəl/ 01 a. Unsubstantial; not real or strong. 1. Unsubstantial; not real or strong.[R.]