Rossi's Theorem is a result in complex analysis that deals with the behavior of holomorphic functions. It states that if a holomorphic function is defined on a domain and is constant on a subset of that domain with a certain property, then the function must be constant throughout the entire domain. This theorem highlights the strong connection between local behavior and global properties of holomorphic functions. The theorem is named after the mathematician Giovanni Rossi, who contributed to the understanding of complex functions. Rossi's Theorem is significant in the study of complex analysis and has implications in various fields, including mathematical physics and engineering.