Entry 1 sense Webster, 1913 Plagioclase /plāj'-ē-ək-lāsˌ/ · Pla·gi·o·clase · IPA /ˈpleɪ.d͡ʒi.əˌkleɪs/ 01 n. A general term used of any triclinic feldspar. See the Note under Feldspar. 1. A general term used of any triclinic feldspar. See the Note under Feldspar.(Min.) See: Feldspar