An application for something such as a job or membership of an organization is a formal written request for it.

The application of a rule or piece of knowledge is the use of it in a particular situation.

In computing, an application is a piece of software designed to carry out a particular task.

Application is hard work and concentration on what you are doing over a period of time.

The application of something to a surface is the act or process of putting it on or rubbing it into the surface.