if not likelihood
What does it mean?
e.g.
"So, if you miss important information as the dialogue or discourse proceeds, there is the danger, if not likelihood, that you may 'lose the thread' of what the dialogue or discourse is about."

Question

Answer

In this sentence you can think of "danger" as being the same as "possibility." The author is saying that there is DEFINITELY the 'danger' or 'possibility' that you will lose the thread, and that it is even LIKELY to happen.

Answer

It's not a phrase, its more of an abbreviated sentence. 
you can read it as '...there is the danger, if not a danger, then a likelihood, that you may...'
In other words - the writer is saying that they think there is a danger, but if not that, or they are challenged on that, then at least there is a likelihood.