The word bardo literally means "an interval between two things." Bar means 'interval' and do means 'two'. We can think of this interval in a spatial or temporal way. If there are two houses, the space between them is a bardo. The period between sunrise and sunset, the interval of daylight, is a bardo.
 Even in the case of the momentary thoughts that arise in our mind, there is an interval between one thought arising and fading and the next thought appearing. Such a gap is part of every process. Everything we experience has this quality of intervals between states.

The interval of unconsciousness into which the mind is plunged by the trauma of death, is called 'Bardo between Birth and Death' ('che shi bar do').

                                                           Kalu Rinpoche