A good which is purchased alongside another good as the combined outcome helps to satisfy a want or desire.
Below is a set of diagrams to illustrate joint demand in a demand and supply framework. In this instance an increase in demand for printers, similtaneously raises the demand for ink cartridges. This is because no individual can use a printer without accompanying ink cartridges so there has to be a joint demand for this product. The same intuition holds for razors and razor blades.