n. 25B6; adjective
his prime reason for leaving : MAIN, chief, key, primary, central, principal, foremost, first, most important, paramount, major; informal number-one.
the prime cause of flooding : FUNDAMENTAL, basic, essential, primary, central.
prime agricultural land : TOP-QUALITY, top, best, first-class, first-rate, grade A, superior, supreme, choice, select, finest; excellent, superb, fine; informal tip-top, A1, top-notch.
a prime example : ARCHETYPAL, prototypical, typical, classic, excellent, characteristic, quintessential.
secondary, inferior.
25B6; noun he is in his prime : HEYDAY, best days/years, prime of one's life; youth, salad days; peak, pinnacle, high point/spot, zenith.