Undercover is a term used to describe activities that are concealed or kept hidden from public view. Some synonyms for undercover include covert, stealthy, sneaky, surreptitious, disguised, hidden, secretive, undercover, and incognito. These words can be used to describe a wide range of activities such as undercover police operations, secret investigations, spy work, and even going incognito on social media. Each of these synonyms have their own unique connotations and nuances, so it is important to choose the right word depending on the context and tone of the text.