A consumer's desire to buy something and the ability to pay for it is called:

A. demand

B. supply

C. economic desire

D. law of diminishing returns

This desire is called the "demand" - answer A.

For example, if in a given context there are a lot of consumers that are interested in buying a certain kind of a computer, one can say that there is a great demand for this computer. Also, if noone wants to buy it, then there is no demand for it.