Entry 1 sense Webster, 1913 Planarian /plən-ĕr'-ē-ən/ · Pla·na·ri·an · IPA /pləˈnɛ.ɹi.ən/ 01 n. One of the Planarida, or Dendrocœla; any turbellarian worm. 1. One of the Planarida, or Dendrocœla; any turbellarian worm.(Zool.)