Subpixel - what is it?
Images are technically limited to the original pixel resolution. Objects in an image are spatially quantized at this resolution. However, an edge for instance may be localized "between" native pixels. Traditional tools like basic gradients often stick to that initial resolution. However, using additional information (multiple data, motion vectors, prior), one can infer a more accurate object location with a sub-pixel resolution, i.e. with an accuracy finer than that of the pixel width, at a fraction of the pixel size, with non-integer coordinates.

This can also apply to track/detect objects with a size smaller than that of a pixel. See for instance: What is a subpixel?

Using clever subpixel shifts, one can for instance reveal asymmetry not visible in the original image, as shown below:



Old and recent references:


Image Sequence Enhancement Using Sub-pixel Displacements, 1988
Multi-frame image super-resolution reconstruction using sparse co-occurrence prior and sub-pixel registration, 2013


Note: subpixels is also used in subpixel rendering on displays, and refers to colored pixel components (red, green, blue, plus  yellow, white  or cyan) that are displayed, or viewed, like a single pixel.