When economists talk about "supply" and "demand," they use the term to refer to the whole curve--supply (or demand) as a function of price.  They also talk about the "quantity supplied" at a particular price.  In an ordinary market situation, you have an upward sloping supply curve (suppliers will supply more at a higher price) and a downward sloping demand curve (consumers will demand less at a higher price) and a point where they cross--when the quantity supplied and the quantity demanded are equal at a particular price:  the market-clearing price.

A whole lot of what economicsts do has to do with the shape of those curves:  Do they slope steeply, or are they relatively flat?  Is the whole curve shifting out or back?