Etymologies
The word diaeresis is from Ancient Greek: διαίρεσις, meaning division, separation, distinction.
Umlaut is German for "around/changed" (um-) "sound" (Laut).
The word trema (plural tremas or tremata), a term occasionally used in classical scholarship, is from Ancient Greek: τρῆμα, meaning a perforation, orifice, or pip (as on dice), and is derived from the form of the diacritic rather than its function.
Read more about this topic: Diaeresis (diacritic)