Which type of airspace is primarily defined as not being A, B, C, D, or E?

Class G airspace is defined as uncontrolled airspace, which means it does not fall under the classifications of A, B, C, D, or E airspace. This airspace is typically found at lower altitudes, where there is less traffic and fewer regulatory requirements. In Class G airspace, pilots operate under visual flight rules (VFR), and air traffic control (ATC) does not provide separation services, allowing for more freedom in flight operations.

Controlled airspace, which includes classes A, B, C, D, and E, is regulated by ATC to ensure safe and orderly flow of aircraft traffic. Special use airspace serves specific purposes such as military operations or hazardous activities and is also governed by restrictions. Therefore, the unique characteristic of Class G airspace being unchecked by ATC and not classified under the other airspace classes solidifies its position as the correct answer.