What defines a Synchronous Message in a sequence diagram?

A Synchronous Message in a sequence diagram is defined by its requirement for an immediate response. This type of message indicates that the sender must wait for the receiver to process the message and return a response before it can continue its operation. This synchronous interaction ensures that the communication between objects is tightly coupled, meaning the sender and receiver are actively engaged in the communication process.

In contrast to asynchronous messages, where the sender can continue its work without waiting for a response, synchronous messages emphasize the necessity of waiting for an acknowledgment or reply. This characteristic is crucial in scenarios where the timing of responses affects the overall flow of the operation or functionality being modeled in the sequence diagram.

This immediate feedback loop helps maintain consistency and control over the interactions in the system, particularly in situations where subsequent actions depend on the information or confirmation received from the receiver.