Definition
A data stream is a time-ordered sequence of events produced continuously and delivered to those who have subscribed. The difference from a file transfer is not primarily speed, but who takes the initiative: the recipient waits instead of asking. Apache KafkaDistributed event streaming platform - widely used for real-time data pipelines and event-driven architectures. is the most widespread example of a platform built for this, where events are placed on a topic and read by several independent consumers at their own pace.
The stream is ordered and verifiable. Each event has its place in the sequence, and a consumer that has been down can catch up from where it left off without the sender having to resend. That distinguishes a data stream from plain notification, which says that something happened but not what.