Energy-efficient communication is a research field and academic concept focused on minimizing the energy consumed in the transmission and reception of data across communication systems. It investigates methodologies, protocols, and hardware designs that optimize the ratio of transmitted data to energy expended, addressing the imperative to reduce power usage. This field explores techniques such as backscatter communication, ambient energy harvesting integration, and resource allocation strategies, aiming to balance communication performance metrics like data rate, latency, and reliability against energy cost. Its significance lies in enabling sustainable connectivity, supporting energy-constrained devices like ambient Internet of Things (IoT), reducing environmental impact, and facilitating the development of future network paradigms like 6G by improving energy sustainability and operational efficiency.