Why is the definition between "quantity demand" and "demand" so important?
The definitions are so important because they are different things. When I am saying quantity demand, I am saying the amount of units consumers in a market are willing to purchase at a given price. When I say demand, I am referencing the the quantities being demand at various price points. Quantity demanded will change when price changes. Demand changes when something happens that makes individual consumers in a market desire the good more or less, causing them to want higher or lower quantities of a good or service at every price.