Reciprocity principle – within the sphere of Law of Contracts, this legal term refers to the principle of fair mutual benefit, mostly expressed between parties who have concluded a contract. In practice, for example with concluded property purchase agreement, the seller receives sale price for his property. On the other hand, the buyer receives a property against the money they are paying. So the benefits are reciprocity. Every party has received something against performance of their obligations.
See also “Law of Contracts”