In chemistry and geology, a solidus is the set of temperatures below which a substance is completely solid or crystallized. The solidus defines the temperature at which a substance begins to melt, but not necessarily when the substance is completely melted. A solidus may be contrasted with a liquidus. The solidus and liquidus may not be the same in all cases. When a difference exists between the solidus and liquidus temperatures, then in the intervening region, the system consists of a mixture of solid and liquid phases. In eutectic systems, solidus and liquidus temperatures are the same: The mixture melts completely at the eutectic point.