I agree that the term is pretty vague. I use it mainly as the opposite of the term "extremist". And that's kinda the point i think, it does not clearly define a set of political opinions, but rather describes any moderate position with a general willingness to compromise and cooperate, a commitment to democracy, as well as condemnation of political violence.

So while in contemporary German politics, I wouldn't equate social democrats with centrists, I would say that they are the most centrist party currently in parliament.

I absolutely don't mind using other terminology, but I do believe that the term "centrist" can be useful sometimes. And after all, this post being about centrists is the reason we are talking about the term in the first place.