a lens like a bifocal but with an additional narrow area ground to adjust the intermediate focus for objects at c. 76 cm (c. 30 in)
See trifocal in American Heritage Dictionary 4
(trī-fōˈkəl, trīˈfōˌ-)
adjective
Having three focal lengths.
Having one section that corrects for distant vision, a second section that corrects for medium vision, and a third that corrects for near vision, as an eyeglass lens.