High-definition video is video of higher resolution than is standard. While there is no specific meaning for high-definition, generally any video image with more than 480 horizontal lines (North America) or 570 lines (Europe) is considered high-definition. 720 scan lines is generally the minimum even though many systems greatly exceed that.
Source:
http://en.wikipedia.org/wiki/High-definition_video
HD refers to definition, not bitrate.
That's why some advertise as HD vs HD with high bitrate.