What is Multiple Listing Service?

Legal Definition
A multiple listing service (MLS, also multiple listing system or multiple listings service) is a suite of services that enable real estate brokers to establish contractual offers of compensation (among brokers), facilitates cooperation with other broker participants, accumulates and disseminates information to enable appraisals, and is a facility for the orderly correlation and dissemination of listing information to better serve broker's clients, customers and the public. A multiple listing service's database and software is used by real estate brokers in real estate (or aircraft broker in other industries for example), representing sellers under a listing contract to widely share information about properties with other brokers who may represent potential buyers or wish to cooperate with a seller's broker in finding a buyer for the property or asset. The listing data stored in a multiple listing service's database is the proprietary information of the broker who has obtained a listing agreement with a property's seller.
-- Wikipedia
Legal Definition
A marketing organization of member brokers agree to co-own their listing agreements with each other to improve procuring more quickly than they could on their own ready, willing, and able buyers for their listings. Exclusive right-to-sell or exclusive agency listings are accepted from their member brokers by most multiple listing services.