Eclipse Link is an open-source Object-Relational Mapping (ORM) framework and Java Persistence API (JPA) implementation. It is developed by the Eclipse Foundation and provides functionalities to map Java objects to relational database tables and vice versa.

Key features of Eclipse Link include:

1. Object-Relational Mapping (ORM): Eclipse Link allows developers to define mappings between Java classes and database tables. This simplifies data access and eliminates the need for writing complex SQL queries manually.

2. Java Persistence API (JPA) Support: Eclipse Link is a reference implementation of the JPA specification, which is a standard for accessing, managing, and persisting data between Java objects and relational databases.

3. Caching: Eclipse Link provides caching mechanisms to improve performance by reducing the number of database queries and interactions. It includes various cache coordination options for distributed environments.

4. Querying: EclipseLink supports a powerful query language called JPQL (Java Persistence Query Language), which is similar to SQL but operates on persistent entities and their attributes.

5. JPA Criteria API: In addition to JPQL, EclipseLink supports the JPA Criteria API, which allows for creating type-safe, object-oriented queries programmatically.

6. Multi-Tenancy Support: EclipseLink offers features for building multi-tenant applications, where data from multiple tenants (organizations or users) can be securely isolated within the same database.

7. Database Platform Support: EclipseLink supports various relational database management systems (RDBMS) and NoSQL databases, making it flexible for different application requirements.

8. Integration with Java EE and Java SE: EclipseLink can be easily integrated into Java EE applications using Java EE containers like Apache Tomcat, GlassFish, or WildFly. It can also be used in standalone Java SE applications.

9. Advanced Mapping Options: EclipseLink provides advanced mapping options such as support for inheritance mapping, relationship mappings, and converters for custom data types.

10. Performance Optimization: EclipseLink offers several features to optimize database access and transaction management, such as batch fetching, lazy loading, and write-behind caching.

Overall, EclipseLink is a robust and feature-rich ORM framework that provides developers with powerful tools to manage and access data in Java applications. It is a popular choice for Java developers when working with JPA-based applications and has a strong community of users and contributors due to its open-source nature.