In the Unified Modeling Language (UML), an association is a fundamental relationship between classes used to represent a connection or relationship between objects of those classes. An association expresses that objects of the involved classes are somehow connected or related to each other. Associations are a critical component of class diagrams in UML and help model the structure of a system.

Here are some key features and concepts related to associations:

1. Class Participation: An association typically connects two or more classes, often referred to as the "source class" and "target class," establishing a connection between them.

2. Roles: Each class in an association can play a role that describes its specific function in the relationship. For example, an association between the "Student" and "Course" classes might have the roles "Student" and "Course Instructor" to represent the relationship between a student and the course instructor.

3. Multiplicity: Multiplicity specifies how many instances of one class can be associated with an instance of another class. It is often expressed using minimum and maximum values (e.g., 1..* for "one or more").

4. Navigational Direction: Associations can have a navigational direction that indicates which class activates the association and in which direction the relationship can be traversed.

5. Cardinality: Cardinality describes how many instances of one class are related to an instance of another class. It is often represented using numbers or symbols such as "1" (one) or "*" (zero or more).

6. Association Endpoints: Association endpoints are the points where the association connects with the involved classes. They can have roles, multiplicities, and navigational directions.

Associations are used to model relationships between classes and objects in a system. They can represent simple relationships like "a person has an address" or more complex relationships between different classes. Associations are a vital part of modeling software architectures and contribute to improving the understanding and communication of a system's structure.