The term 'criticism, informally' can be summarized by the crossword solution 'FLAK, ' which is a four-letter word. FLAK is a slang term used to describe criticism or negative comments, usually in the form of verbal abuse. It is derived from the German word 'Fliegerabwehrkanone, ' which literally translates to 'anti-aircraft gun.' The term was first used during World War II to describe the barrage of anti-aircraft fire that pilots had to endure. Today, it is commonly used to refer to any kind of criticism or negative feedback.