To become demand, a desire, (i) must be backed by the ability or the capacity to pay for the good, and (ii) the willingness on the part of the consumer to spend for the good. A demand is, thus, an effective desire. Further, a demand which cannot be met due to shortage of commodity is called an insatiable demand.
Demand is always defined with reference to price and a time period. Thus, it is allow concept it is meaningless to specify demand without reference to price and time period. The statement that demands for apples is 2000 kg. is meaningless.
The price at which these apples are demanded is to be mentioned, as with the change in price, the quantity demanded may also change. Demand is also expressed with reference to time. Even at the same price, demand may change, depending upon the time period under consideration.
Thus, at Rs. 10 per kg. demand for apples may be different at different times during a particular period. Demand for a good may be defined as the, “quantity of a commodity that will be bought at a particular price and during a given period or point of time.”