Is there a description of Prolog language (syntax and semantics) available online?
There are a lot of reference manuals for implementations. But neither of those is a language description. For example the SWI Prolog manual states
This manual does not describe the full syntax and semantics of Prolog.
And refers to a set of books printed on paper, published in the nineteen eighties. And to ISO standard which is for money and "should be available from my country's ISO representative" gibberish.