In the past, and still to a degree presently, a Litvak was a Jew from Lita or one with Litvish roots. However, as there are very few Jews in Lita today, and as there are not that many Jews around any more anywhere that grew up in Lita, a new definition of Litvak is emerging.
The label Litvak today is changing to mean someone who is Litvish in outlook - who shares Litvish approaches to life, Torah and the like - even if that person may have no ancestors that lived in Lita. To be a citizen of the spiritual Lita - the Lita shel maalah - of the Vilna Gaon, Rav Yisroel Salanter, the Beis Halevi, etc., one just has to share their values - even if their roots are in Kiev, Cracow, Tunis, Sanaa, Budapest, Casablanca, Aleppo, Riga, or Berlin.
We are still in a transitional period between the old definition and the new one, but the new one is steadily gaining ground, in my opinion.
Lita - not just a place - but a way of life.