The phrase deep field is a small survey field for surveys carrying out lengthy observations to identify very distant phenomena, e.g., galaxies and quasars at high redshift. The observation time can be days or more, typically focused on a region of no more than a square degree, often much less. Often the field (or a portion of it) is subsequently used for other surveys, to cover more spectra, to add depth, and/or to follow up with spectrography.
The 1995 Hubble Deep Field is the best known and perhaps the originator of the term. Other examples of deep fields and surveys fixed on deep fields: