A domestic airport is an airport that primarily serves a country's internal flights. A domestic flight is one where the destination and origin are both located within the same country. In some countries, like Australia, New Zealand and Canada, a "domestic" flight may actually cross international boundaries to reach its destination. But in most other cases it will not.
For example, flights between London Heathrow Airport and Birmingham Airport in England are classified as international flights because Heathrow's main operations are outside of England (and all operations for Birmingham Airport are inside England). Such flights may still carry a “domestic” label because they can connect to wider domestic networks or routes operated by airlines from the same country.
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