Pigment spot ocellus refers to a small, circular, pigmented spot found on marine organisms such as jellyfish or crustaceans. These spots serve as a rudimentary eye, detecting changes in light and shadow. While pigment spot ocellus is the accepted scientific term, there are a few synonyms used to describe the same phenomenon. One such term is simple eye, which highlights the basic function of these spots. Another synonym is pseudopupil, which refers to the appearance of the spot when viewed from different angles. Lastly, the term eyespot is commonly used to describe pigmented spots found on single-celled organisms such as euglena or chlamydomonas.