HD stands for High Definition.
For video, high definition means support for higher video resolutions. For TV, HD usually means 1920×1080 pixels, while in video calling, HD usually means 1280×720 pixels.
For voice, high definitions means wideband.
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The WebRTC Glossary is an ongoing project where users can learn more about WebRTC related terms. It is maintained by Tsahi Levent-Levi of BlogGeek.me.