Time, often described as the fourth dimension, is a phenomenon which we experience in linear progression, in the direction of increasing entropy and chaos.
That is to say, that the amount of disorder in the Universe is ever increasing, as the stars slowly cool and die out, some exploding into massive clouds of gas and dust which coalesce into new stars which once again either cool and die out or explode into clouds of gas and dust, cycling again into new stars.
Some suggest that Time is a human concept, and that the direction of causality is simply that which we experience, and that the true direction of time is in the opposite direction, not toward increasing entropy, but toward decreasing entropy, in which the total disorder in the Universe is actually decreasing as the Universe contracts in the direction opposite causality, while we as humans experience things backward.
Of course, this is not demonstrated, and is merely an interesting theory which gives me the willies every time I think about it on the toilet.