Down is the direction in which an object will travel assuming that it is more dense than that which surrounds it and has sufficient weight. In outer space (or any other place with no gravity) down does not exist.
A down is a hill, usually made of chalk and in southern England. See downland. This is the original meaning of the word, and the direction comes from an old word adown meaning 'from the hill'.
Down is the negative direction along the z-axis. Up and down are the only two well-defined directions in the presence of gravity.
Down in railway parlance is the traditional term for the direction leading from the principal terminus, i.e. away from milepost zero. For details of UK usage, see "up".