There isn't a formal definition of a roadmap, mainly because roadmaps are used to communicate information, and the amount of information that people want to show inside a roadmap varies depending on the working environment and the target audience.
A roadmap should show the WHY and the WHAT of your next goals (Where you are headed? What are the things on your path to get there? Why is that important?).
They might look like this for example:
But many of them end up looking like this:
A lot of tools show them similar to a Gantt chart because they are easy to draw, and because it's easy to attach dates to the boxes. Much more so than with a free floating format as in the first images.
This is where the lines between a roadmap and a timeline start to become blurry and you end up with a mix of two concepts:
- your next goals, and
- the outputs needed to implement those goals within a given time frame.
So now it starts to become about the HOW and the WHEN.
This tends to happen because most tools facilitate it, and also because the first question you always get asked when looking at any roadmap, is "When will it be ready?"
A roadmap can include time frames for sure, but they are usually in quarters or months. If you end up with things like "the 15th of September", then you might not just be looking at a roadmap.
I'm not sure what kind of a roadmap your supervisor wants, but since the last model is the most commonly used, and since he told you to make two different versions of the roadmap (for a team of 2 and a team of 4), I'm suspecting it's this last one. Maybe as far as turning the roadmap into a real Gantt chart to show a breakdown of the work, with sequence of activities, dependencies, and assigned deadlines (i.e. "If we have 2 people working on it, how long will it take? If we had 4 people working on it, how much sooner could it be ready?").
The best way to find out your supervisor's expectations would be to ask. Maybe he has some older examples of what he expects that he can share and you could look at. Like I said at the beginning, roadmaps are visualizations used to communicate information, so find out what kind of information your supervisor wants to extract from the roadmap and you will then know how to build one and what to add to it.