[1870–75; tri- + apsidal]This word is first recorded in the period 1870–75. Other words that entered Englishat around the same time include: asymmetric, billing, immobilize, linkage, upgradetri- is a combining form meaning “three,” used in the formation of compound words. Otherwords that use the affix tri- include: triglyceride, triglyph, trilemma, trinomial, triple