Y-PSNR is PSNR measured on the luma channel alone — the Y (brightness) component — as opposed to the colour channels or a combined average. Video is split into brightness (luma) and colour (chroma, the U and V components), and PSNR is computed separately for each, so a tool reports psnr_y, psnr_u, psnr_v, and a combined psnr_avg. The human eye is far more sensitive to brightness than to colour, which is why encoders spend most of their bits on luma; for the same reason the luma PSNR is the figure most engineers quote, because it tracks the part of the picture viewers actually notice. The combined average is weighted by how many pixels each plane holds, and since chroma is usually subsampled, luma dominates it anyway. The practical catch is ambiguity: a bare PSNR figure with no component named could be luma or average, so reporting should always say which — and Y-PSNR, while the most useful single value, will miss a chroma-only artifact entirely.
