What is a project and what is not?

When explaining the fundamentals of project management to managers, it is a challenge to specify what should be a project and what should not. Sometimes, it is hard for managers to discern between a task and a project. Some tasks require extensive planning. A rule of thumb that I use is that if you have five or more related tasks, they are probably better of being viewed as a small project. The probability of success is higher is these tasks are viewed as a project. The number of 5 used in my rule of thumb is not scientific. However, it has worked well for me in terms of defining projects.