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.""