That's in my opinion what reliable / unrealiable mean in this context.

reliable = it either succeeds or you get an error after some time, unreliable = it may or may not succeed

I concur with the people who think "best effort" is not a good term. But perhaps TCP streams are not reliable enough for TCP to be rightly called a reliable stream protocol. As it turned out, it's not really possible to use TCP without a control channel, message chunking, and similar mechanisms for transmitting arbitrary large files. If it really offered reliable streams that would its primary use case.