Market Data Definition Language (MDDL) is an XML-based interchange format and common data dictionary on the fields needed to describe 1) financial instruments, 2) corporate events affecting value and tradability and 3) market-related, economic and industrial indicators.

The principal function of MDDL is to allow entities to exchange market data by standardizing formats and definitions. MDDL provides a common format for market data so that it can be efficiently passed from one processing system to another and provides a common understanding of market data content by standardizing terminology and by normalizing the relationships of various data elements to one another.

The goal of Market Data Definition Language (MDDL) is to define a publicly available standard that provides a generic XML-based interchange format on the fields needed to describe financial instruments (including identifiers and current/historical values), corporate events (including specific corporate and instrument information affecting value and tradability), and market-related information (including economic and industrial indicators). The goal is to promote data interoperability.

• MDDL focuses on broad field definitions for market data content, applicable to multiple classes.
• The initial constructs of MDDL are for snapshot and time series applications. MDDL can be extended to historical, streaming and interpretative and vendor-specific data models as appropriate.
• Decisions regarding the MDDL standard must be ratified by consensus of the MDDL Steering Committee with a 2/3 majority quorum.
• Intellectual property rights in MDDL are owned by the Financial Information Services Division (FISD) of the Software & Information Industry Association (SIIA), but freely licensed and made available on an "as is" basis.

Market data user firms distribute a significant amount of pre-defined market data to a wide variety of database applications and spend a significant amount of time and money translating market data formats and modifying applications for internal communication.

From the user perspective, the goal of MDDL is to enable users to integrate data from multiple sources by standardizing both the input feeds used for data warehousing (i.e. define what's being provided by vendors) and the output methods by which client applications request the data (i.e. ensure compatibility on how to get data in and out of applications). The basis of the user requirement is for MDDL to:

1. Support multiple vendors.
2. Provide a common vendor interface (identical query and delivery protocol to all vendors).
3. Allow for a common request format for different vendors with standard request types and field names. MDDL must provide a standard snap request by vendor specific instrument names or by vendor independent ISIN, exchange code, instrument type (or any combination).
4. Be extensible and accommodate vendor and exchange specific fields.
5. Support entitlement and permissioning requirements.
6. Standardize and normalize data field names across multiple data sources.
7. Provide a mechanism for validation and data quality.
8. Support the automation and security identification requirements of STP
9. Provide standard for common request/response interface for retrieving the data.