What word describes actions kept or done in secret, often to conceal an illegal purpose?

The term "clandestine" specifically refers to actions that are carried out in secret, typically because they involve illicit or nefarious purposes. This word encapsulates the idea of hidden activities that are intentionally concealed from others, often to avoid detection or scrutiny. In contexts where legality and ethics are concerned, clandestine actions imply a deliberate effort to keep something under wraps, indicating potential wrongdoing or moral ambiguity.

In comparison to the other terms provided, "open," "transparent," and "ostensible" all suggest qualities of clarity or visibility, which contrast sharply with the secretive nature implied by "clandestine." While these alternatives might describe actions or behaviors that are honest or straightforward, they do not carry the same connotation of secrecy and illegality that defines clandestine actions. Therefore, "clandestine" stands out as the word that accurately conveys the essential meaning related to secrecy for illicit purposes.