Generally, transiting through an airport is not considered "entering" a country in the legal sense, provided you remain in the international transit zone (airside). In this zone, you have not passed through immigration or customs, and you are essentially in a legal "no-man's-land." However, this varies significantly by country. In the United States and Canada, there is no concept of a "sterile" international transit; all passengers arriving from abroad must clear immigration and customs, even if they are immediately departing for another country. In the Schengen Area, you can transit between two non-Schengen countries without a visa at most major hubs (like Frankfurt or Paris), but if your connection requires you to change terminals or if you are moving between two Schengen countries, you must formally enter the region. Always check if you require a Transit Visa (like the Direct Airside Transit Visa in the UK) based on your nationality, as being "airside" does not always exempt you from visa requirements.