Substitute 替代品
Good which can be replaced by another to satisfy a want.
替代品是指两种产品存在相互竞争的销售关系，即一种产品销售的增加会减少另一种产品的潜在销售量，反之亦然。

Elastic demand 弹性需求
Where the price elasticity of demand is greater than 1. The responsiveness of demand is proportionally greater than the change in price. Demand is infinitely elastic if price elasticity of demand is infinity.
弹性需求是指当产品或服务的价格有所变动时，市场对该产品或服务的需求也发生明显变动的状况。

Inelastic demand 非弹性需求
Where the price elasticity of demand is less than 1. The responsiveness of demand is proportionally less than the change in price. Demand is infinitely inelastic if price elasticity of demand is zero.
需求价格弹性绝对值小于1的情况。在这种情况下，当价格下降时，总收益下降；当价格上升时总收益上升。无价格弹性的需求的一个极端是完全无价格弹性的需求，这意味着当价格上升或下降时，需求量完全没有变化。

Price elasticity of demand or own elasticity of demand 需求的价格弹性
The proportionate response of changes in quantity demanded to a proportionate change in price.
表示需求量对价格变动反应程度的指标。弹性系数等于需求量变动百分比除以价格变动的百分比。

Unitary elasticity 单位弹性
Where the value of price elasticity of demand is 1. The responsive of demand is proportionally equal to the change in price
介于有价格弹性的需求和非价格弹性的需求之间的一种情况。价格弹性的绝对值恰好等于l。

Cross or cross-price elasticity of demand 需求的交叉弹性 
A measure of the responsiveness of quantity demanded of one good to a change in price of another good. It is measured by dividing the percentage change in quantity demanded of one good by the percentage change in price of the other good.
需求的交叉弹性是指商品Y的价格发生1％的变化时所引起的商品X的需求量变化的百分比。

Income elasticity of demand 需求的收入弹性
A measure of the responsiveness of quantity demanded to a change in income. It is measured by dividing the percentage change in quantity demanded by the percentage change in income.
需求的收入弹性是指，当价格保持不变时消费者收入发生1％的变化时，所引起的需求数量变化的百分比。