Greetings.

In a computing context, I've always taken 'semantics' to refer to everything 'above' the level of 'syntax', in a cheerfully hand-waving way.

Thus

  2 + 2

is a legal string of arithmetic tokens, in the syntax we learned in primary school, and

  2 2 +

isn't.  Choosing to interpret one or both of these token-streams as an arithmetic expression is a semantic act.  If we've heard of reverse-polish notation, then we may additionally recognise the second stream as a legal stream in that syntax, in which case we will ascribe to the second stream the same semantics -- the same meaning/implication/truth-value/blah -- as the first.