Cloaking
Cloaking means serving robots content different from what visitors see — text added for engines, selective redirects. It ranks among the most severely penalised violations in Google’s spam policies.
In practice
Detection is straightforward for an engine that also crawls using standard browser profiles. This is distinct from legitimate adaptations: declared geolocation, language-based content, paywalls marked up per guidelines. The deciding criterion is intent to deceive the ranking system, not the technical difference itself.