Some other important words, which recur from time to time in the
Yoga-sutras, need to be understood, though there are no exact
English equivalents. As they must be used to avoid clumsy
circumlocutions, it is necessary to explain them. It is said:
"Yoga is Samadhi." Samadhi is a state in which the consciousness
is so dissociated from the body that the latter remains
insensible. It is a state of trance in which the mind is fully
self-conscious, though the body is insensitive, and from which
the mind returns to the body with the experiences it has had in
the superphysical state, remembering them when again immersed in
the physical brain. Samadhi for any one person is relative to his
waking consciousness, but implies insensitiveness of the body. If
an ordinary person throws himself into trance and is active on
the astral plane, his Samadhi is on the astral. If his
consciousness is functioning in the mental plane, Samadhi is
there. The man who can so withdraw from the body as to leave it
insensitive, while his mind is fully self-conscious, can practice
Samadhi.

The phrase "Yoga is Samadhi" covers facts of the highest
significance and greatest instruction. Suppose you are only able
to reach the astral world when you are asleep, your consciousness
there is, as we have seen, in the Svapna state. But as you slowly
unfold your powers, the astral forms begin to intrude upon your
waking physical consciousness until they appear as distinctly as
do physical forms, and thus become objects of your waking
consciousness. The astral world then, for you, no longer belongs
to the Svapna consciousness, but to the Jagrat; you have taken
two worlds within the scope of your Jagrat consciousness--the
physical and the astral worlds--and the mental world is in your