In mathematics, the quantity on the y-axis (vertical axis) is referred to as the dependent variable and the quantity on the x-axis is referred to as the independent variable. However, the placement of price and quantity on the axes is somewhat arbitrary, and it should not be inferred that either of them is a dependent variable in a strict sense.
This site uses the convention that a lowercase q is used to denote individual demand and an uppercase Q is used to denote market demand. This convention isn’t universally followed, so it’s important to always check whether you are looking at individual or market demand. (You will be looking at market demand in most cases.)