Within a number of MAVIS support articles the terms 'scanlines' and 'horizontal samples' are used frequently and it is therefore useful to have an understanding of what they mean.
A frame of video is a digital image made up from a number of pixels. The number of pixels used in the formation of this digital image is called the spatial resolution. Most people will likely have come across spatial resolutions when thinking of 1080 (HD) or 2160 (UHD).
'1080 (HD)', for example, is really shorthand for the spatial resolution of a video. This denotes that the video has 1080 pixels vertically by 1920 pixels horizontally.
When talking about spatial resolution, to help us easily determine whether we are referring vertical or horizontal sets of pixels (and for a number of legacy reasons) we have different terms for each.
Each row of pixels is called a scanline. Taking the example from above, we would say that 1080 (HD) has 1080 scanlines. For each scanline, we would then say that it has 1920 horizontal samples.
Using this terminology allows us to reference a specific pixel within the image. For example, Line 45, Sample 6 gives an exact coordinate to an individual pixel within the image.
The term 'scanline' is really a legacy term from the days of old analog televisions. To form an image on a CRT television a electron beam would make a number of sweeps across the display to form a picture. The electron beam would "scan" the screen to produce the image on the display. The old analog television spec had to define the number of these sweeps, or scanlines, so that each television could render the broadcast signal correctly. For NTSC television it look 525 scanlines to form a picture, for PAL it was 625.
Within raster graphics, the term scanline is used, by analogy, to describe a signal row of pixels.