Mostowski is a mathematical concept named after the Polish mathematician Andrzej Mostowski. It primarily refers to the Mostowski collapse lemma, which is a technique used in set theory. This lemma helps in understanding the structure of certain sets by showing how they can be simplified or transformed into a more manageable form. In addition to the collapse lemma, Mostowski's work includes contributions to model theory and descriptive set theory. His research has influenced various areas of mathematics, particularly in understanding the foundations of set theory and the relationships between different mathematical structures.