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An activity in Unified Modeling Language (UML) is a major task that must take place in order to fulfill an operation contract. The Student Guide to Object-Oriented Development defines an activity as a sequence of activities that make up a process. Activities can be represented in activity diagrams

An activity can represent:

The invocation of an operation.
A step in a business process.
An entire business process.

Activities can be decomposed into subactivities, until at the bottom we find atomic actions.

The underlying conception of an activity has changed between UML 1.5 and UML 2.0. In UML 2.0 an activity is no longer based on the state-chart rather it is based on a Petri net like coordination mechanism. There the activity represents user-defined behavior coordinating actions. Actions in turn are pre-defined (UML offers a series of actions for this).

Unified Modeling Language

Actors

Organizations
- Object Management Group
- UML Partners
Persons

Concepts

Object oriented	Object-oriented programming Object-oriented analysis and design Object-oriented modeling
Structure	Actor Attribute Artifact Class Component Interface Object Package Profile diagram
Behavior	Activity Event Message Method State Use case
Relationships	Association Composition Dependency Generalization (or Inheritance)
Extensibility	Profile Stereotype
Other	Multiplicity

Diagrams

Structure	Class Component Composite structure Deployment Object Package
Behaviour	Activity State Machine Use case
Interaction	Communications Sequence Interaction overview Timing

Derived languages