KSI measures pressure and stress; it stands for kilopounds per square inch or one thousand units of pounds per square inch. It is chiefly used to express the relative pressure to vacuum as compared to atmospheric pressure.
KSI is almost exclusively used in the fields of engineering and architecture to measure large amounts of stress on materials; it may also be used to represent tensile strength. In all cases, KSI refers to pressure from tangible, physical objects. Engineers will sometimes use KSI in place of "pounds per square inch" when communicating with engineers in other parts of the world, as it does not require converting between the metric and imperial systems.