The meaning and behavior of programming language constructs - what the code actually does when executed, as opposed to just how it's written. Semantics defines the relationship between syntax and the program's actual behavior.
From Greek 'semantikos' meaning 'significant' or 'meaning,' from 'sema' (sign, meaning). Adopted from linguistics in the 1960s when computer scientists needed to distinguish between code structure (syntax) and code meaning (semantics).
Semantics is the difference between saying something correctly and saying something meaningful. You might write syntactically perfect code like 'divide by zero' - the grammar is right, but the meaning makes no sense! It's like saying 'The purple elephant danced on Tuesday' - perfect grammar, but questionable meaning.
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