There's no standard definition of a "speed number" in common usage. It could be used in a few different contexts, but without more information, it's difficult to say exactly what you're looking for.

Here are some possibilities:

* Speed in a specific context: Someone might refer to a "speed number" as a specific speed limit, or a maximum speed allowed for a particular vehicle or activity. For example, "The speed number for this highway is 65 mph."

* Codeword: In some industries or contexts, a "speed number" could be a codeword used to refer to a specific speed, possibly for security or privacy reasons.

* Mathematical term: In mathematics, there are concepts like the "speed of convergence" which refers to how quickly a sequence approaches a limit. This might be loosely called a "speed number" in a specific situation.

To understand what "speed number" means, please provide more context! For example:

* What is the context of the conversation where you heard this term?

* What are you trying to learn about speed numbers?

* Can you give an example of how this term is used?

With more information, I can give you a more precise answer.