Hydraulic Cartridge

What the heck is a hydraulic cartridge?

After much research, I still don't know the what, why, or how of a hydraulic cartridge. Sounds like something used in the building of planes to me. Or maybe it's something used on cranes. You know, a part responsible for the lifting or moving of something?

Anyone out there have any idea? Because if no one can tell me, I'm going to keep on guessing, and it will drive me insane until I know.

Maybe hydraulic cartridges are used on the Space Shuttle? "Houston, we have a problem. Our hydraulic cartridge has malfunctioned and we're returning home."

I don't know, but it sounds like they perform an important job.