A diacritic consisting of two dots ( ¨ ) placed over a letter, used among other things to indicate umlaut or diaeresis.
From Ancient Greek τρῆμα (trêma, “hole”), from τετραίνω (tetraínō, “perforate”), used for the dots on dice, via Dutch trema and French tréma.
Practice speaking and memorizing "trema" and many other words and sentences in English.