1] A desire to possess a commodity – A desire is a wish to acquire something, it may or may not be accompanied by the effective means to fulfill it. He may or may not be able to buy it.
2] Willingness to spend – A misers desire to install a central air cooling system in his house cannot become demand as he lacks the willingness to pay though he has the ability to pay.
3] Ability to purchase the commodity (depends on disposable income and the price of the commodity). E.g. a poor persons desire to purchase a rolls Royce cannot become demand as he has the willingness to pay for it but not have the ability to pay.
4] Availability of a good or service – There can be no demand in absence of availability of the commodity even if the consumer is willing and able to pay for it.
Therefore all desires cannot become demand
5] Demand is a flow, relative to the amount at a price in a particular time period for a group of products.
Demand is a is the amount purchased at a particular price , in a particular time period, (depending on age, seasons, etc), our willingness and ability to pay is subjected to a time period. A day, a week etc. It is a flow concept. Demand is usually for a group of products, not for a single product. It is also in relation to place and persons (depending on likes and dislikes, circumstances, etc). E.g. sweets, demand for sugar for a family, at Rs 20/- per kg for a month is 5 kg.
Therefore according to:-
Benham “The demand for anything at a given price, is the amount of it, which will be bought per unit of time at that price.”
Bobber, “By demand we mean the various quantities of a given commodity or service which consumers would buy in one market in a given period of time at various prices”
Normally when price falls some existing consumers buy more and new consumers entre the market Leading to a rise in demand. When price rises some existing consumers buy less and some consumers leave the market causing a fall in demand. This indicates that there is an inverse relation between price and quantity demanded.