Q.What do you mean by 'change in demand'?

'Change in demand' means the whole demand curve shifts (due to a non-price factor changing), as opposed to a movement along a fixed demand curve caused by a change in the good's own price.

Economists distinguish carefully between two related but different ideas:

• Change in quantity demanded: a movement ALONG a given, unchanged demand curve, caused ONLY by a change in the price of the good itself (other things — income, tastes, prices of related goods — held constant).
• Change in demand: a shift of the ENTIRE demand curve to a new position, caused by a change in any of the non-price determinants of demand, while the good's own price is held constant. These determinants include: consumer income, tastes and preferences, prices of related goods (substitutes and complements), consumers' expectations about future prices, and the number of buyers in the market. …

The law of demand says quantity demanded varies inversely with price, other things constant; the total price effect on quantity demanded can be split into the substitution effect and the income effect.

Main stem -- State the law of demand: The law of demand states that, ceteris paribus (with the consumer's income, tastes, prices of related goods, and all other influences held constant), the quantity demanded of a commodity is inversely related to its own price -- as the price of a good falls, consumers demand more of it, and as the price rises, they demand less. This inverse relationship is represented graphically by a downward-sloping demand curve, and it holds for most normal goods (excepting special cases such as Giffen goods).

Or-alternative -- Two components of the price effect: When the price of a good changes, the resulting total change in quantity demanded (the 'price effect') can be decomposed into:

1. Substitution effect -- the change in quantity demanded that results purely from the good becoming relatively cheaper (or dearer) compared with other goods, inducing consumers to substitute towards (or away from) it, holding the consumer's real satisfaction/utility level constant. …

Demand = desire backed by willingness and ability to pay, at a given price, in a given time period. (Or) An inferior good is one whose demand decreases as income increases.

Demand for a commodity: In economics, demand means more than merely wanting a good — it is the quantity of a commodity that a consumer is both willing and financially able to buy at a particular price, during a specific period of time (e.g. per day, per month). Mere desire unsupported by purchasing power is not 'demand' in the economic sense.

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The law of demand is the inverse price-quantity relationship holding other factors constant; a Giffen good is the rare inferior good that violates it because its income effect overwhelms its substitution effect.

Law of demand. Keeping income, tastes and the prices of related goods unchanged, the quantity demanded of a commodity is inversely related to its own price: a rise in price lowers quantity demanded, and a fall in price raises it. This is why an ordinary demand curve slopes downward. Two forces drive it: the substitution effect (a costlier good is replaced by cheaper alternatives) and, for most goods, an income effect that works in the same direction (a price fall raises real purchasing power, letting the buyer afford more).

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Demand is willingness-cum-ability to buy at a price; an inferior good is one whose demand moves opposite to income.

Demand for a good

In everyday language 'demand' simply means desire for a good. In economics, demand is a much narrower idea: it is the quantity of a good that a consumer is both WILLING to buy and ABLE to buy (has the purchasing power for) at a given price, during a specified time period, other things (income, tastes/preferences, prices of related goods) remaining unchanged. A mere wish to own a good, without the money to back it, is not demand - it is only a desire. Demand is also always expressed with reference to a price and a time period (e.g. '10 kg of rice per month at Rs. 40/kg'), because the same consumer will demand different quantities at different prices (the law of demand: price and quantity demanded are inversely related, other things equal).

Or - Inferior good …

A Giffen good shows a direct price-quantity relationship instead of the usual inverse one; Giffen goods and Veblen (prestige) goods are the two most cited exceptions to the law of demand.

Giffen good. The law of demand states that, other things remaining the same, quantity demanded falls as price rises. A Giffen good is the classic exception: it is a strongly inferior good that occupies a large share of a poor consumer's budget (typically a staple food). When its price rises, the consumer's real income falls so sharply that they can no longer afford even small quantities of superior substitutes, and are forced to buy MORE of the now-costlier staple just to meet basic needs. Here the (large) negative income effect outweighs the (normal) negative substitution effect, so the net effect of a price rise is a RISE in quantity demanded. The name comes from Sir Robert Giffen, who observed 19th-century English labourers buying more bread as its price rose, because they could no longer afford meat.

Or — two exceptions to the law of demand:

1. Giffen goods — demand rises with price, as explained above. …