Since the launch of ChatGPT in 2022, there has been growing interest in the potential role of artificial intelligence (AI) in second language teaching. AI offers unique advantages, such as its ability to cater to individual learning differences and function as a 24/7 support system for both students and instructors. However, before fully integrating AI tools into classroom settings, it is crucial to assess their effectiveness in enhancing language acquisition. This synthesis study addresses this gap by systematically reviewing 113 empirical studies published between 2004 and 2024 that examine the use of AI in second language learning. The study seeks to answer two synthesis questions: (1) What linguistic knowledge and skills have AI tools been used to enhance in second language instruction? (2) What types of AI tools have been utilized in second language instruction? By analyzing these studies, we aim to provide educators and researchers with insights into how AI can be most effectively implemented in language education, ensuring that it supports teaching effectiveness and meets diverse learner needs. The findings will inform both theoretical discussions and practical applications of AI in second language instruction.