Alfred Tarski was a Polish-American logician and mathematician, born in 1901 and known for his work in formal logic and semantics. He made significant contributions to the field of mathematical logic, particularly through his development of the concept of truth in formal languages, which has influenced various areas of philosophy and linguistics. Tarski's most notable achievement is the Tarski's undefinability theorem, which states that truth cannot be consistently defined within the same language. His work laid the groundwork for later developments in model theory and philosophy of language, making him a key figure in 20th-century logic and mathematics.