The term 'expert, informally' can be summarized by three crossword solutions: MAVEN (5 letters), WHIZ (4 letters), and Meister (7 letters). A MAVEN is someone who has a deep understanding of a particular subject or activity. A WHIZ is someone who is very skilled at something, often due to their experience and knowledge. Finally, a meister is a master of a particular craft or skill. All three terms are informal ways of referring to an expert in a certain field.