Round-trip time is the duration measured from when a signal is sent to when its corresponding acknowledgement is received back at the origin, capturing the combined effect of propagation delay, processing delay, and queuing along the path. It is a standard diagnostic for network latency and is used to estimate achievable throughput, retransmission timers, and interactive responsiveness. Round-trip time grows with physical distance because of propagation delay but is also affected by congestion and routing.