Q:

Flash - Definition of a Component?

is there a standard rule or best practice which dictates when a custom class/library should be a used/distributed as a component or remain simply as a class/library?
what's the definition of a component?

A:

I often use component as synonymous with "Class".  Any class is a component.  
Many people refer to components, specifically, as UI Components.  In the context of Flex, this is probably any component that extends UIComponent.
You should put a component it in a separate class library if you want to use the component across multiple projects.  Or when you want to distribute the component to other developers independent of an application.  
In some cases, you may consider using components for parallel development purposes, so that  two developers can build different pieces of the same application in isolation of each other.  

