Used somewhat loosely, the term "bardo" refers to the state of existence intermediate between two lives on earth. According to Tibetan tradition, after death and before one's next birth, when one's consciousness is not connected with a physical body, one experiences a variety of phenomena. These usually follow a particular sequence of degeneration from, just after death, the clearest experiences of reality of which one is spiritually capable, to, later on, terrifying hallucinations arising from the impulses of one's previous unskilful actions. For the spiritually advanced the bardo is a state of great opportunity for liberation, since trancendental insight may arise with the direct experience of reality, whilst for others it can be a place of danger as the karmically created hallucinations can impel one into a less than desirable rebirth. This is described in the Bardo Thodol (Bardo Thos-grol), (lit. "liberation through hearing in the intermediate state) the so-called Tibetan Book of the Dead . This work actually differentiates the intermediate states between lives into three bardos (themselves further subdivided): the chikhai bardo or "bardo of the moment of death", the chonyid bardo or "bardo of the experiencing of reality", and the sidpa bardo or "bardo of rebirth". The chikhai bardo is the stage of experiencing the "clear light of reality", or at least the nearest approximation to it of which one is spiritually capable. The chonyid bardo is the stage of experiencing visions of various Buddha forms (or, again, the nearest approximations of which one is capable), whilst the sidpa bardo is the stage of karmically impelled hallucinations which eventually result in rebirth. It is interesting to compare the descriptions of the Bardo Thodol with accounts of certain "out of the body" experiences described by people who have nearly died in accidents or on the operating table - these typically contain accounts of a "white light", experienced as, somehow, a living being, and of helpful figures corresponding to that person's religious tradition.
The Bardo Thodol also mentions three other bardos: those of "life" (or ordinary waking consciousness), of "dhyana", and of "dream". Thus together the "six bardos" form a classification of states of consciousness into six broad types, and any state of consciousness is a type of "intermediate state" - intermediate between other states of consciousness. Indeed, any momentary state of consciousness can be considered a bardo, since it lies between our past and future existences; it provides us with the opportunity to experience reality, which is always present but obscured by the projections and confusions due to our previous unskilful actions.
In the West, the term bardo is also used to refer to times when our usual way of life is suspended, as, for example, when we are on retreat. Such times can be fruitful for spiritual progress as there are few external constraints, although they can be very challenging because our unskilful impulses can come to the fore, just as in the sidpa bardo.