Meridian (plural: "meridians") is used in
perimetry and in specifying
visual fields. According to IPS Perimetry Standards 1978 (2002):
"Perimetry is the measurement of [an observer's] visual functions ... at topographically defined loci in the visual field. The visual field is that portion of the external environment of the observer [in which when he or she is] steadily fixating ...[he or she] can detect visual stimuli."