the limitations on consumption of different goods imposed by the fact that house holds have only a limited amount of money to spend (their budget). The budget constraint defines the opportunity set of individuals, when the only constraint that they face is money

For an individual or household, the condition that income equals expenditure (in a static model), or that income minus expenditure equals the value of increased asset holdings (in a dynamic model). For a country, the condition that the value of exports equals the value of imports or, if capital flows are permitted, that exports minus imports equals the net capital outflow. It is equivalent to income from production equaling expenditure on goods plus net acquisition of foreign assets. The curve, usually a straight line, representing either of these conditions.

The limits to expenditure.