In law, a promise or undertaking, founded on a consideration. This promise may be verbal or written; An assumpsit is express or implied; express, when made in words of writing; implied, when in consequence of some benefit or consideration accruing to one person from the acts of another, the law presumes that person has promised to make compensation. In this case, the law, upon a principle of justice, implies or raises a promise, on which an action may be brought to recover the compensation. Thus if A contracts with B to build a house for him, by implication and intendment of law, A promises to pay B for the same, without any express words to that effect.

An action founded on a promise. When this action is brought on a debt, it is called indebitatus assumpsit, which is an action on the case to recover damages for the non-payment of a debt.