A transmission medium is the physical pathway through which data travels from one point to another. It can be a wired medium, like copper cables or fiber optic cables, or a wireless medium, such as radio waves or microwaves. The choice of medium affects the speed, distance, and quality of the data transmission. Different transmission media have unique characteristics. For example, fiber optic cables can transmit data over long distances with minimal loss, while copper cables are more susceptible to interference. Understanding these differences helps in selecting the appropriate medium for specific communication needs.