Dropouts are a means of identifying candidates for distant
galaxies through photometry.
A dropout is the instance of a source showing in some
passbands but not others.
A high redshift will cause some of the galaxy's
typically-strong electromagnetic radiation (EMR) bands (e.g., that at the high point
of its black body spectrum) to be shifted to the degree
that the images taken in some bands do not show the galaxy at all.
Given a set of bands, a galaxy that shows in bands with
frequencies up to, but not including that of an R band
can be termed a red dropout galaxy.
Similarly for a blue dropout galaxy.
The term OIR-dark galaxy is also used, for those that
show in neither optical nor near infrared.
The advantage of this method of searching for distant galaxies is that
a few images of a single field of view provide information on multiple
galaxies within that field of view,
hinting as to which ones might be extremely distant,
reducing the necessary spectrography to identify examples.