Viscera are the internal organs of the body — heart, lungs, liver, stomach, intestines — taken together, especially those in the chest and belly. It is a plural noun and behaves like one: the viscera are, not is. The singular, viscus, exists but is rare outside medical writing.

The word is technical and clinical. Doctors, biologists, and anatomy textbooks use it; ordinary people say "organs", "insides", or in a butcher's context "guts" or "offal". So when viscera turns up in non-medical writing, it is usually there for effect — the cold precision of the word makes a scene of injury or dissection feel more exact and more disturbing.

The related adjective visceral has drifted a long way from the body and is the form learners meet far more often. A visceral reaction is one you feel in your gut before you think it through — instinctive, physical, not reasoned. "I had a visceral dislike of the place." That sense is now the everyday one; a doctor might still use visceral in the literal anatomical sense, but almost nobody else does.