A bytestream is a series of bytes.
Formally, a byte stream is a certain abstraction, a communication channel down which one entity can send a sequence of bytes to the entity on the other end. Such channel is often bidirectional, but sometimes unidirectional. In almost all instances, the channel has the property that it is reliable; i.e. exactly the same bytes emerge, in exactly the same order, at the other end.
Less formally, one can think of it as a conduit between the two entities; one entity can insert bytes into the conduit, and the other entity then receives them. This conduit can be ephemeral or persistent.
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