How are airways primarily defined within airspace?

Airways are primarily defined as corridors of airspace that connect Terminal Control Areas (TCAs). This definition highlights the purpose of airways, which facilitate the organized flow of air traffic between different regions, particularly in busy areas around airports where TCAs are located. Airways are established to enable safe and efficient navigation for aircraft, allowing them to follow designated pathways that enhance traffic management and reduce the risk of collisions.

In this context, Airways serve multiple functions, including providing structure to air traffic, ensuring separation between aircraft, and serving as reference points for navigation for pilots. These corridors accommodate a wide range of aircraft, both commercial and private, under controlled environments, particularly around key terminal areas.

While the other options touch on aspects of air navigation, they do not accurately capture the main definition and purpose of airways. Fixed routes at lower altitudes might describe specific procedures but don't encompass the broader definition involving TCAs. Similarly, airways are not limited to military use or designed specifically for weather conditions; they cater to all types of air traffic operating in the aviation system.