  Speaker diarization provides the answer to the question "who spoke when?" for
an audio file. This information can be used to complete audio transcripts for
further processing steps. Most speaker diarization systems assume that the
audio file is available as a whole. However, there are scenarios in which the
speaker labels are needed immediately after the arrival of an audio segment.
Speaker diarization with a correspondingly low latency is referred to as online
speaker diarization. This paper provides an overview. First the history of
online speaker diarization is briefly presented. Next a taxonomy and datasets
for training and evaluation are given. In the sections that follow, online
diarization methods and systems are discussed in detail. This paper concludes
with the presentation of challenges that still need to be solved by future
research in the field of online speaker diarization.
