In the Class diagrams, an Association represents the semantic relationship between two or more classifiers which specifies connections between their instances. An Association relationship is the most general of all relationships and the most semantically weak.
In the Use Case diagrams, an Association represents the participation of an Actor in a Use Case, for example, when instances of an Actor and instances of a Use Case communicate with each other. This is the only relationship between Actors and Use Cases. Sometimes an association relationship is called communication Association.
An Association is drawn as a solid path connecting two classifier symbols.