A sound returned by a surface, arriving later and changed. An echo proves both the sound and the room.
Greek ἠχώ (ēkhō, a sound, a returned sound), from ἦχος (ēkhos, noise); via Latin echo into Old French and English. The mountain nymph Echo of Ovid's Metamorphoses, cursed to repeat only the last words spoken to her, is the same word personified — the myth is a story ABOUT the noun, not its source.
The journal realm: what you put in comes back to you in your own words, changed only by time. The Echo of the myth could not speak first — but she never lost a word she was given, which is exactly the sovereignty law the realm keeps.