reading in unison; saying the same thing
A yojijukugo describing a group of people speaking or reading the same words together, often in a chorus or with one voice.
The students read the textbook in unison.
At the meeting, everyone expressed their agreement with one voice.
異口同音 means 'many people saying the same thing independently', while 一口同音 emphasizes simultaneous, coordinated speech or reading.
A yojijukugo composed of 一口 (one mouth) and 同音 (same sound), literally 'one mouth, same sound'. The exact historical derivation is uncertain, but it is conventionally associated with the idea of speaking in unison.