Prana is vital energy, the power of life; the essential substrate of all forms of energy. It is like the wind, in that it cannot be seen, but we know it is there ~ we can detect it by how we feel, like after a yoga class or during a pranayama session.
Some people may experience prana as a feeling of well-being like feeling the warm sun on your face, others may experience prana in other ways, possibly as a bright, white light, an impression of something flowing, a tingly sensation, a warm feeling or perhaps in some other way.
In Sanskrit, pra means first, na means vibration and an means to breath.
Prana is the fundamental vibratory life force and is apparent through awareness of the breath.
Eknath Easwaran poetically describes Prana here in the translation of the Prashna Upanishad:
“Prana burns as fire;
He shrines as the sun;
He rains as the cloud;
He blows as the wind;
He crashes as thunder in the sky.
He is earth;
He has form and no form;
Prana is immortality.
Everything rests in Prana, as spokes rest in the hub of a wheel”
“Prana is the force which exists in all things…although closely related to the air we breathe; it is more subtle than air or oxygen”