Well, if we do, those would be very different dictionaries indeed. Do you not know "forecast" is a synonym to "prediction" ? I was going to qualify this with a "practically", but then it turns out according to Mirriam-Webster, prediction is literally a synonym of "forecast". So explain again, what exactly is your "dictionary definition" for "prediction" again? Unless it's guessing, but then, anyone can then make "predictions" like that.

(If we stick to Mirriam-Webster again, here is what I found : to calculate or predict (some future event or condition) usually as a result of study and analysis of available pertinent data - i.e. - basically what I already told you a "prediction" is).

Is this perhaps some weird British vs American English thing I was unaware of?

Oxford Learners Dictionary (because the Oxford English Dictionary is behind a paywall): "a statement that says what you think will happen; the act of making such a statement" https://www.oxfordlearnersdictionaries.com/us/definition/eng...

The current Wikipedia definition looks like a good fit for how I'm using the term here:

A prediction (Latin præ-, "before," and dictum, "something said"[1]) or forecast is a statement about a future event or about future data. Predictions are often, but not always, based upon experience or knowledge of forecasters. There is no universal agreement about the exact difference between "prediction" and "estimation"; different authors and disciplines ascribe different connotations. https://en.wikipedia.org/wiki/Prediction