Income, refers to consumption opportunity gaine by an entity within a specified time frame, which is generally expressed in monetary terms. Usage of the term may, however, be somewhat ambiguous. For households and individuals, "income is the sum of all the wages, salaries, profits, interests payments, rents and other forms of earnings received... in a given period of time." For firms, income generally refers to net-profit: what remains of revenue after expenses have been subtracted. In the field of public economics, it may refer to the accumulation of both monetary and non-montary consumption ability, the former being used as a proxy for total income.