What does the Internet term "jack" mean?

The jack is a steel lifting member as a working device, through the top of the bracket or the bottom of the bracket claw in the stroke to lift the weight of the light lifting equipment. According to the structural characteristics can be divided into rack and pinion jack, screw jack and hydraulic jack 3 kinds. Jack is a small lifting height (less than 1m) of the simplest lifting equipment. It has a mechanical and hydraulic two kinds. Mechanical jack and rack and spiral two kinds, due to the lifting capacity of small, laborious operation, generally only used for mechanical maintenance work, in the process of repairing the bridge is not applicable. Hydraulic jacks are compact, smooth, self-locking, so they are widely used. The disadvantage is that the lifting height is limited, the lifting speed is slow.