What is discipline?
To act according to a standard of Truth or a rule or law of action (dharma) or in obedience to a superior authority or to the highest principles discovered by the reason and intelligent will and not according to one’s own fancy, vital impulses and desires. In Yoga obedience to the Guru or to the Divine and the law of the Truth as declared by the Guru is the foundation of discipline.
Sri Aurobindo – Letters on Himself and the Ashram: CWSA, Vol. 35, p. 672
Without discipline no proper work is possible.
Without discipline no proper life is possible.
And above all, without discipline no Sadhana is possible.
The Mother – Words of the Mother – I: CWM, Vol. 13, p. 163
Sadhak = one who follows a yogic discipline.
Sadhana = yogic discipline.
Yoga = union with the Divine (by extension: the path which leads to this union).
The Mother – Words of the Mother – II: CWM, Vol. 14, p. 32