Which term describes actions that are hidden or undercover?

The term that best describes actions that are hidden or undercover is "covert." This word is used to refer to activities that are intentionally concealed and not meant to be observed or discovered by others. Covert actions are planned to ensure that they remain secret, which is crucial in contexts such as intelligence operations, military actions, or any activities where stealth is required.

While "secretive" and "disguised" can imply a level of concealment, they do not inherently carry the same connotation of planned or intentional secrecy as "covert." "Clandestine" also suggests hidden operations but often implies that the actions are illegal or morally questionable. Thus, "covert" stands out as the term specifically associated with intentional concealment without the additional implications that might come with the other options.