The taking of readings from a single data source. In basic CCD imaging theory, for delivered optical system information not to be lost, the resolution of imaging CCD pixels must be at least twice as precise as the delivered resolution of the optical system (e.g., for appropriate sampling, a telescope delivering a PSF with an FWHM of 2 arcseconds calls for a CCD with pixels having a one-arcsecond FOV.) Less than this level of precision is termed undersampling. Significantly more is termed oversampling.
Sampling refers to the proper matching of pixel size to focal length to achieve the best possible resolution. Sampling is the essentially number of pixels that a star image covers. If too few, the image is undersampled and does not achieve the best possible resolution. If too many, the image is oversampled which unnecessarily reduces efficiency.
Sampling, re-sampling, scaling, up-sampling, down-sampling, and resizing up or down . . . terms used for increasing or decreasing the number of pixels in an image. Resizing down is easier to accomplish as existing pixels are simply discarded.