The principle of brahmacarya is celibacy. There are two kinds of brahmacaris. One is called naisthika-brahmacari, which means one who takes a vow of celibacy for his whole life

Expressions researched:
"The principle of brahmacarya is celibacy. There are two kinds of brahmacārīs. One is called naiṣṭhika-brahmacārī, which means one who takes a vow of celibacy for his whole life,"

SB Canto 3

There are two kinds of brahmacārīs. One is called naiṣṭhika-brahmacārī, which means one who takes a vow of celibacy for his whole life, whereas the other, the upakurvāṇa-brahmacārī, is a brahmacārī who takes the vow of celibacy up to a certain age. For example, he may take the vow to remain celibate up to twenty-five years of age; then, with the permission of his spiritual master, he enters married life.

SB 3.22.14, Translation and Purport:

Svāyambhuva Manu continued: O wise man, I heard that you were prepared to marry. Please accept her hand, which is being offered to you by me, since you have not taken a vow of perpetual celibacy.

The principle of brahmacarya is celibacy. There are two kinds of brahmacārīs. One is called naiṣṭhika-brahmacārī, which means one who takes a vow of celibacy for his whole life, whereas the other, the upakurvāṇa-brahmacārī, is a brahmacārī who takes the vow of celibacy up to a certain age. For example, he may take the vow to remain celibate up to twenty-five years of age; then, with the permission of his spiritual master, he enters married life. Brahmacarya is student life, the beginning of life in the spiritual orders, and the principle of brahmacarya is celibacy. Only a householder can indulge in sense gratification or sex life, not a brahmacārī. Svāyambhuva Manu requested Kardama Muni to accept his daughter, since Kardama had not taken the vow of naiṣṭhika-brahmacarya. He was willing to marry, and the suitable daughter of a high royal family was presented.