Sometimes, you find that you are creating something repeatedly and, similar to a configuration template, you need a template to generate objects of a given type. Some examples of this might be Apache virtual hosts, a specific type of application, or anything else that is repeated a lot. This is where definitions come in, and they are stored in the definitions directory inside of a cookbook.
Definitions are loaded and available as named resources just as other resources such as packages, files, and so on are; the only difference is that there is no provider. You can think of them as resources and providers all in one. Subsequently, they are much more rigid and limited in scope than a normal resource would be. Here is an example definition to install Python libraries using pip and a requirements.txt file:
define :pip_requirements , :action => :run do
name = params[:name]
requirements_file = params[:requirements_file]
pip = params[:pip]
user = params[:user]
...