Well, in DL-land, it needs to be reported and evaluated, then included in the database. It has a slightly different nuance than "unknown" or what "I don't know" since it emphasizes personal contact with something.
First of all I would say that "Bedeutung" in German comes closest to "meaning" in meaning in general. E.g. the "meaning of a word" would be "die Bedeutung eines Wortes". Without context this is how one would in most cases understand the word.
Now, "Bedeutung" can also mean "significance" in the sense of "importance", but then you usually talk about "Bedeutung von etwas" ("significance of something"), e.g. of an event. If you then wanted to say that you were unfamiliar with the fact that said event had such a significance, you would rather say something like "Mir war unbekannt, dass dieses Ereignis eine solche Bedeutung hatte." in German. "Diese Bedeutung ist mir unbekannt." could have the same meaning, if everything was clear from context, but it would still sound very unnatural. Although I must say that the English sentence "This significance is unfamiliar to me." also seems unnatural in a similar way in my opinion.
Right, and in English academic usage, interestingly enough, we probably would say "I don't know the meaning of this."
Why is 'definition' wrong (as in ' This definition is unfamiliar to me')? Definition is a synonym of meaning, is it not?
Often, dative is used when the English expression is "to [something]".
Es ist mir kalt = "It is cold to me"
Ich gebe ihm ein Buch = "I give a book to him"
Unfortunately this doesn't work for all situations where you use dative though.