From these, practice is the effort to be fixed in concentrating the mind.

Bryant Commentary:

Patañjali now describes the first ingredient required to restrain the mind, abhyāsa, practice, and defines practice as the effort to concentrate the mind. Vyāsa, in turn, defines concentration (one-pointedness) as the peaceful flow of the mind when it has become freed from its fluctuating states or vṛttis. The effort to secure this state is practice. It is important to recognize that a controlled mind is not going to manifest by itself: Effort, yatna, is required. One is reminded here of the comment of Arjuna (who, as a warrior, had no history of serious yoga practice), that the mind is “harder to control than the wind” since it is “fickle, powerful, and obstinate.” As noted by Vijñānabhikṣu in the last sūtra, Kṛṣṇa assures him that without doubt the fickle mind is hard to control, but it can be subdued with “practice and dispassion” (VI.34–35). In this sūtra, Patañjali indicates that this practice requires effort, and Vyāsa associates this effort with enthusiasm and vigor.

Vyāsa also uses the term sādhana, which typically refers to one’s specific daily spiritual practices. In the context of classical yoga, Vācaspati Miśra understands this sādhana to be the eight limbs of yoga that will be discussed in Chapters II and III. So the vṛttis of the mind can be restrained when one is enthusiastic, vigorous, and steadfast in the practice of these eight steps.

The commentators reiterate that one can only hope to be concentrated or one-pointed—the mind can only flow peacefully—when its rājasic and tāmasic potential have been stilled. Practice is the effort involved in attaining this end. Rāmānanda Sarasvatī introduces the next sūtra by wondering how practice can ever achieve steadfastness, since it is constantly disturbed by rājasic and tāmasic saṁskāras, distracting memories and tendencies inherited from time immemorial, a question with which aspiring yogīs can surely relate.

Exertion To Acquire Sthiti Or A Tranquil State Of Mind Devoid Of Fluctuations Is Called Practice.

Practice is the sustained effort to rest in that stillness.

Of these two, practice is the continuous struggle to become firmly established in the stable state of the True Self.

Abhyasa is the effort of being firmly established in that state (of CittaVrtti-Nirodha).

Taimini Commentary:

What is Abhyasa? All effort directed towards the attainment of that transcendent state in which all Citta-Vrttis have been suppressed and the light of Reality shines uninterruptedly in its fullest splendour. The means of attaining this objective are many and various and all these may be included in Abhyasa. It is true that in the particular system of Yoga put forward by Patanjali only eight kinds of practices have been included and hence it is called Astanga-Yoga, i.e. Yoga with eight component parts. But there are other systems of Yoga prevalent in the East and each has its own particular technique. Many practices are common but there are some which are peculiar to each system. Patanjali has included in his system practically all those which are essential or important. The Yogi can adopt any of these according to his needs or temperament though he is generally advised to confine himself to those practices which are prescribed in the particular school to which he belongs.

It need hardly be pointed out that Yoga is an experimental science and as in all sciences new techniques are being constantly discovered by individual teachers and taught to their disciples. Thus each advanced teacher, though he follows the broad principles of Yoga and techniques of his particular school, imparts a personal touch to his teachings by introducing certain minor practices of his own. These are generally continued in a more or less modified form as long as the school lasts. But in most cases such schools quickly degenerate into mere academic bodies carrying on a dead tradition.

Practice is the steadfast effort to still these fluctuations..

Iyengar Commentary:

Practice is the effort to still the fluctuations in the consciousness and then to move towards silencing it: to attain a constant, steady, tranquil state of mind.

In order to free the mind from fluctuations and oscillations and to reach a state of steadiness, the practitioner is advised to practise intensely all the yogic principles, from yama to dhyana. These embrace all disciplines: moral, ethical, physical, mental, intellectual and spiritual. (For the application of the mind to the practice, see 1.20.)”

Of these two, effort toward steadiness of mind is practice.

Satchidananda Commentary:

 Here Patañjali means continuous practice, not just for one or two days. You have to always be at it, not just for a few minutes a day and then allowing the mind to have its own free time all the other hours. It means you become eternally watchful, scrutinizing every thought, every word and every action. How? Patañjali gives three qualifications:

Continuous struggle to keep them (the Vrttis) perfectly restrained is practice.

SV Commentary:

What is this practice? The attempt to restrain the mind in the Chitta form, to prevent its going out into waves.

Steadiness is the undisturbed calmness of the flow of the mind, when it has become free from the modifications.

Effort to secure that end is the putting out of energy to secure, and aspiration towards that.

Practice is the resort to the means thereof with the object of attaining it`

~ Rāma Prasāda translation.