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What is High Definition video or High Definition Television?

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1. A simple answer: HD TV and Video has more pixels than regular, or standard, video formats. Think of it like this: If you drew a picture using 100 dots then drew the same picture using 1000 dots, the one with 1000 dots would be clearer; pixels are like dots.
2. "Common" is almost there... pixels are not the important thing when it comes to video resolution. Pixels are important for stills, though... For video, horizontal lines of resolution are what count. 480 horizontal lines is standard definition video. Anything greater than this is "high definition". This can be either 720 horizontal lines or 1080 horizontal lines. There are some cameras like the Red ONE and Silicon Graphics systems that record at higher horizontal line counts - also referred to a "ultra high definition".
3. the term HD developed when the ATSC set the standards for digital television. television defines picture formats based on the number of scanning lines. in fact old CRT displays actually worked by scanning the beam from left to right, one line at a time. in modern displays the "scan" is more virtual. anyway ATSC formats that call for 720 and 1080 active picture scan lines are HD. outside of the TV studio, all HD is compressed. Compression schemes all work on one common factor, they require the horizontal pixel count and the scan lines to be a multiple of 16. so a digital picture is better thought of as made up of 16x16 pixel macro blocks. when reception is bad, you can actually see the macro block errors. 1080 is not a multiple of 16, so 1080 is actually 1088 scan lines, the top and bottom 4 lines are encoded but not displayed. This is true for both television and camcorder video.