Autonomous, encapsulated units within a system that provide one or more interfaces.
Per VisualParadigm’s online tutorials for Component Diagrams:
In UML, Components are made up of software objects that have been classified to serve a similar purpose. Components are considered autonomous, encapsulated units within a system or subsystem that provide one or more interfaces. By classifying a group of classes as a component the entire system becomes more modular as components may be interchanged and reused.
Components may be Classes, but also groups of classes.
Components are represented by the same symbol in UML and Archimate - the rectangle with two rectangles on the left:
