In both pronunciations, the emphasis is on the second syllable, which is pronounced with a long "a" sound, though the quality of this vowel sound differs slightly between the American and British versions.
The word "cicada" comes from the Latin "cicada," meaning "tree cricket." The term entered English around the mid-14th century. The root word in Latin reflects the insect's habitat (often trees) and its noisy nature, which likely inspired the name. The consistent feature across languages and time has been the association with the distinctive, loud sound these insects make, which is impossible to ignore and has been noted in literature and common observation alike.