In the classical definition stream is "sequence of data elements", without any additional guarantees. In this context TCP is stream transformer/filter: stream of "raw" packets is transformed into payload stream.

> What do you mean exactly? UDP is not a stream protocol, it is an unreliable datagram one. A single read will return exactly one UDP datagram.

This is exactly what makes UDP a stream of datagrams. Data is defined over one single UDP datagram - UDP gives exactly zero guarantees about data across datagram boundaries. There is no first, there is no last, there is no next/previous datagram, there no guarantees and expectations whether more datagrams will follow.

> Of course QUIC builds a reliable stream abstraction on top of it, but that's not different from TCP.

Key word "reliable", which is subset of streams. In goes stream of datagrams, out goes ordered (and otherwise identified) stream of bytes. Reading your comment it seems that you equate "stream" with "asynchronously fetched buffer" meaning from C++ and others, which is a stricter definition of streams.