Definition
The visible line outside the aircraft where the earth's surface appears to meet the sky, used by the pilot as the primary visual reference for controlling the airplane's pitch and bank attitude in visual flight.
Plain English
The line you see out the windshield where the ground (or sea) meets the sky. Pilots use this line as their main reference for keeping the airplane level and pointed the right way.
Context Anchor
Used in attitude flying when looking outside the airplane to control pitch and bank by reference to the real world, rather than by looking only at instruments.
Derivation
Natural' here means 'occurring in nature' -- the real horizon outside the aircraft, as opposed to the artificial horizon shown on the attitude indicator inside the cockpit. The contrast between the two terms is the whole point of the word 'natural.'
Why Pilots Care
It provides the essential visual reference for attitude control in VFR conditions, allowing pilots to fly without sole reliance on instruments.
Intuition Check
Do not assume the natural horizon is always clear, level, or easy to use. It is the outside horizon as it appears to you, and it can be hidden or misleading in poor visibility, darkness, or uneven terrain.
Example Sentence 1
The instructor told the student to glance at the natural horizon and keep the cowling a fixed distance below it to hold level flight.
Example Sentence 2
During the turn, the pilot kept the wings level with the natural horizon to maintain coordinated flight.