The process of segmenting an audio recording by speaker — answering the question "who spoke when?" Diarization is what lets a meeting transcript show "Alice: Let's ship it. Bob: Not yet." rather than an undifferentiated stream of words. Good diarization is harder than it looks: speakers interrupt each other, audio quality varies across mic setups, and accents and prosody throw off the speaker-embedding models that drive most diarization systems. State-of-the-art systems combine acoustic embeddings (what does this voice sound like) with speech-recognition signals to assign each utterance to a speaker label. Once labels are stable across the meeting, they can be mapped to real names using the participant list from the calendar invite or a one-time enrollment voice sample.