"Kitab al-Ayn" is a comprehensive Arabic lexicon written by the scholar Al-Khalil ibn Ahmad al-Farahidi in the 8th century. It is notable for being one of the earliest dictionaries in the Arabic language, focusing on the roots and meanings of words. The work is organized alphabetically by the letters of the Arabic alphabet, with a particular emphasis on the letter "Ayn." The dictionary serves as a crucial resource for linguists and scholars studying Arabic linguistics and etymology. It not only provides definitions but also explores the phonetic and morphological aspects of words, making it an essential reference for understanding the richness of the Arabic language.