An unincorporated association of two or more persons or entities formed to carry on as co-owners of a business for profit. Partnerships are governed by the laws of the state in which they are formed and the partnership agreement. Certain multi-member LLCs, trusts, and non-US entities are treated as partnerships for US federal income tax purposes.

A contractual arrangement (where no separate entity is formed) may be deemed a partnership if the parties carry on a trade, business, financial operation, or venture and divide its profits (regardless of the intent of the parties or whether the parties entered into a partnership agreement).