A value is optimal if it is the best by some measure or optmisation criterion.
Used in Chap. 4: pages 38, 46, 49; Chap. 7: page 99; Chap. 8: pages 107, 108; Chap. 9: pages 121, 122, 125, 126, 130; Chap. 10: page 141; Chap. 19: page 306
Also known as optimality