An expense is an item requiring an outflow of money, or any form of fortune in general, to another person or group as payment for an item, service, or other category of costs. For a tenant, rent is an expense. For students or parents, tuition is an expense. Buying food, clothing, furniture, or an automobile is often referred to as an expense. An expense is a cost that is "paid" or "remitted", usually in exchange for something of value. Something that seems to cost a great deal is "expensive". Something that seems to cost little is "inexpensive". "Expenses of the table" are expenses for dining, refreshments, a feast, etc.

In accounting, expense is any specific outflow of cash or other valuable assets from a person or company to another person or company. This outflow is generally one side of a trade for products or services that have equal or better current or future value to the buyer than to the seller. Technically, an expense is an event in which a proprietary stake is diminished or exhausted, or a liability is incurred. In terms of the accounting equation, expenses reduce owners' equity. The International Accounting Standards Board defines expenses as:

...decreases in economic benefits during the accounting period in the form of outflows or depletions of assets or incurrences of liabilities that result in decreases in equity, other than those relating to distributions to equity participants.

Expense is a term also used in sociology, in which a particular fortune or price is sacrificed voluntarily or involuntarily by something or someone to something or somebody else, often in the context that the latter is taking advantage of the former.