B.G 8.11

यदक्षरं वेदविदो वदन्ति विशन्ति यद् यतयो वीतरागाः। यदिच्छन्तो ब्रह्मचर्यं चरन्ति तत्ते पदं सङ्ग्रहेण प्रवक्ष्ये ॥११॥

yadakṣaraṁ vedavido vadanti viśanti yad yatayo vītarāgāḥ। yadicchanto brahmacaryaṁ caranti tatte padaṁ saṅgraheṇa pravakṣye ॥11॥

[यत् (yat) - that which; अक्षरम् (akṣaram) - the imperishable; वेदविदः (vedavidaḥ) - knowers of the Vedas; वदन्ति (vadanti) - declare; विशन्ति (viśanti) - enter; यत् (yat) - which; यतयः (yatayaḥ) - ascetics; वीतरागाः (vītarāgāḥ) - who are free from attachment; यत् (yat) - which; इच्छन्तः (icchantaḥ) - desiring; ब्रह्मचर्यम् (brahmacaryam) - the discipline of celibacy and self-restraint; चरन्ति (caranti) - practice; तत् (tat) - that; ते (te) - to you; पदम् (padam) - the supreme state; सङ्ग्रहेण (saṅgraheṇa) - briefly, in summary; प्रवक्ष्ये (pravakṣye) - I shall declare;]

That imperishable (reality) which the knowers of the Vedas declare, which the ascetics free from attachment enter, and which, desiring, they practice the discipline of 'brahmacharya' — I shall briefly declare to you the path to that supreme state.

Gīta Tātparya 8.11

'Brahmacharya' - disciplining the mind and faculties in Brahman.

मनाऽदीनां ब्रह्मणि चरणं ब्रह्मचर्यम् ॥११॥

manā'dīnāṁ brahmaṇi caraṇaṁ brahmacaryam ॥11॥

[मनः (manaḥ) - mind; आदीनाम् (ādīnām) - and those beginning with it (i.e., senses, faculties); ब्रह्मणि (brahmaṇi) - in Brahman, the Absolute; चरणम् (caraṇam) - movement, practice, application; ब्रह्मचर्यम् (brahmacaryam) - discipline directed towards Brahman;]

Disciplining the mind and faculties in Brahman is Brahmacharya.