See also: How to manage applications
In Juju, an application is a running abstraction of a charm in the Juju model. It is whatever software is defined by the charm. This could correspond to a traditional software package but it could also be less or more.
An application is always hosted within a model and consists of one or more units.
Interaction between applications is handled by integrations (before Juju v.3.0, relations).
An application has can have resources, a configuration, the ability to form relations (integrations), and actions.