East Slavic Rurikid principality in the Carpathian foreland, formed when Volodymyrko Volodarovych united the Peremyshl and Zvenyhorod lines and moved his seat to Halych (c. 1141); reached its height under Yaroslav Osmomysl (r. 1153-1187), noted for strong boyar/urban political influence (the 'freedom in princes' tradition of inviting and expelling rulers) and close ties to Byzantium, Hungary, and Poland.
Seized by Roman the Great, prince of Volhynia, in 1199, who united it with his own principality to found Galicia-Volhynia — the existing canonical successor record (galicia_volhynia) begins exactly at 1199, a clean handoff.
Dedicated Ukrainian-language history of the Galician-Volhynian principality, the standard specialist reference.
English-language survey covering the Rurikid Rus' principalities including Galicia's rise before the 1199 union with Volhynia, for readers without Ukrainian.