Patanjali's brahmacharya is made up of two words.
Brahman,
Which is the word for the absolute,
The totality,
And acharya.
Acharya meaning teacher or one who is an example of.
So the ultimate meaning of brahmacharya is one who is an example of Brahman.
In this sense,
Any act,
Any choice,
Any decision or movement that represents our innate divine nature is the essence of brahmacharya.
But traditionally,
The word brahmacharya was intended to convey celibacy and the act of abstaining from the release of the sexual energy and potency available within the human being.
While this is a very worthy practice,
Especially in certain contexts,
The deeper connotation of brahmacharya is the conservation of life force and vitality.
Patanjali says this when he says that for one who's established in brahmacharya,
Vitality is gained.
Our capacity to maintain our life force and energy without expelling it unnecessarily,
Without wasting our life force,
Is an essential practice.
That practice of conserving our energy is the practice of staying at home in ourselves,
Directing our energies toward our higher purposes,
Toward our heart,
Toward our innate intelligence.
As we deplete our energies through sense activity,
Through desire,
Through fear,
Through worry,
We experience ourselves depleted,
Fatigued.
So the whole point of brahmacharya is to conserve our life force,
To be able to preserve our vitality and enhance our vitality.