Words have semantics: their meaning. When you study what words actually mean and how those meanings work, you're doing semantic analysis.

A semantic difference is a difference in meaning. If you and a friend are arguing about whether a hot dog is a sandwich, that's a semantic debate: you're really disagreeing about what the word “sandwich” means. Sometimes people dismiss these discussions by saying “that's just semantics,” suggesting the disagreement is only about words, not anything real.

In computer science, semantics describes what code actually does rather than just how it looks. Two programs might look completely different but have the same semantics: they produce identical results.