The term describes an effect rather than a formal legal category. What matters in practice is a combination of features: no public register of beneficial ownership, permitted nominee and bearer arrangements, corporate directors, minimal filing obligations, and slow or conditional cooperation with foreign requests for information.
The picture changes constantly as registers open and close, so the useful analytical question is never “is this a secrecy jurisdiction” but “what specifically can and cannot be established about a company registered here, and how long does it take”.