- 1seg is a mobile terrestrial digital audio/video and data broadcasting service in Japan.
- Service began expertimentally during 2005 and officially on April 1, 2006.
- First mobile phone handsets for 1seg were sold by KDDI to consumers in autumn 2005.
- Terrestrial digital broadcast in Japan (ISDB-T) is designed so that each channel is divided into 13 segments (plus one segment for separating channels). HDTV broadcast occupies 12 segments, and the remaining (13th) one segment is used for mobile receivers. Thus the name, '1seg'.