publications - How is the Scimago journal ranking (Q1-Q4) defined?

What does it mean that a journal is ranked Q1 according to Scimago Journal and Country Rank?

Answer

I couldn't find documentation on the meaning of Q1, Q2, etc. but the guess that it means "first, second etc. quartile in the ranking" seems to be correct. When you look at one of the "widgets" they have for each journal, it says "quartile":

BTW, whether the journal is in the first, second, third, or fourth quartile, the widget will (erroneously) always say "best quartile". This doesn't change the conclusion that Q means quartile, however.