Ink cartridge
A replaceable container of liquid ink for an inkjet printer, often costlier per millilitre than you care to work out.
The cartridge feeds ink to the printhead's nozzles. On HP and Canon the nozzles frequently sit in the cartridge itself, so you replace the printhead with every change; Epson and Brother build the head into the printer and sell plain ink tanks. Many cartridges carry a chip that tracks the level and locks out third-party ink.
The economics are notorious: the printer sells cheap and the ink dear, a model in which the cartridge is the real product. The counter-move has been refillable tank systems and subscriptions like HP Instant Ink. The laser printer's equivalent is toner, the dot-matrix printer's the ink ribbon.