Which term is synonymous with 'airline'?

The term "carrier" is synonymous with "airline" because it specifically refers to a company that transports passengers or goods through the air. In the aviation industry, an airline operates aircraft to provide air transport services, making "carrier" a direct and commonly used synonym.

"Freight service" focuses on the transportation of goods rather than passengers, which distinguishes it from airlines that usually provide both passenger and cargo services. "Aeronautical company" is a broader term encompassing any business involved in aviation, including airframe manufacturers and maintenance providers, which may not necessarily operate flights. A "travel agency" facilitates the booking of travel services but does not operate aircraft or define itself as a transportation provider. These distinctions highlight why "carrier" is the most appropriate synonym for "airline."