Kalmar "biconvex" sifa ce da ke bayyana siffa mai lanƙwasa ko zagaye waje ta ɓangarorin biyu, kamar ruwan tabarau. Musamman, yana nufin sama ko wani abu da yake da madaidaicin ɓangarorin biyu, ma'ana yana kumbura a waje a tsakiya kuma ya gangara zuwa ƙasa zuwa gefuna. A cikin na'urorin gani, ruwan tabarau na biconvex shine ruwan tabarau wanda ya fi kauri a tsakiya fiye da na gefuna, wanda ke haifar da haɗuwa da haske. Prefix "bi-" yana nufin "biyu" ko "duka," kuma "convex" yana nufin mai lanƙwasa waje. Don haka, "biconvex" a zahiri yana nufin "mai lankwasa a waje ta bangarorin biyu."