Up describes the positive z-value in a gravitational field, just as "down" represents the negative z-value. The z-axis is the only axis that is not apparently arbitrary. The x and y values, "North", "South", "East", and "West" do in fact exist (and are named relative to the direction of the Earth's spin) but are not readily apparent.
A computer or machine is said to be up when it's working properly, and down when it isn't.
See also: UP