mid-15c., "protraction, delay," verbal noun from train (v.). From 1540s as "discipline and instruction to develop powers or skills;" 1786 as "exercise to improve bodily vigor." Training wheels as an attachment to a bicycle is from 1953.
Training is the development of the mind or character or both, or some faculty, at some length, by exercise, as a soldier is trained or drilled. Discipline is essentially the same as training, but more severe. [Century Dictionary]
"skilled form of horseback riding performed in exhibitions and competitions," 1936, from French dressage, from dresser "to train, drill" (see dress (v.)). Middle English had dress (v.) in the sense of "to train or break in" a horse or other animal (c. 1400), but it died out.
c. 1300, disciplinen, "to subject to (penitential) discipline, correct, chastise, punish," from Old French descepliner and directly from Medieval Latin disciplinare, from Latin disciplina (see discipline (n.)). Meaning "instruct, educate, train" is from late 14c. Related: Disciplined; disciplines; disciplining.