When an economist says that "Kevin's income elasticity of red wine is 6" he means that if Kevin's income increases by 10%, the quantity of red wine demanded by Kevin rises by 60%. So, red wine is income elastic. Since the income elasticity is greater than 1, red wine is a luxury good for Kevin.
Income elasticity measures the change in the quantity of goods demanded relative to a change in income.
If an increase in income results in a decrease in the quantity of goods demanded, then that good is an inferior or cheap good. The income elasticity of a cheap good is negative.
If the demand for a good rises with an increase in income, then that good is a normal good. The income elasticity of normal goods is greater than zero.
If an increase in income results in a greater increase in the quantity of goods demanded, then that good is a luxury good. The income elasticity of a luxury good is greater than 1.
