Demand is economics means both the willingness as well as the ability to purchase a commodity.
Law of demand
The law of demand states that the quantity demanded of a good or service decreases as it price increases and conversely, the quantity demand increases when the price decreases
Assumptions
Income of people remain unchanged
Taste, preference and habits of consumer unchanged.
Prices of related goods i.e. substitute and complementary goods remain unchanged.
There is no expectation of future change in price of commodity— The commodity in question is not consumed for its prestige value.