Definition
The longest straight line that can be drawn through the center of an ellipse, passing through both foci and connecting the two farthest points on the curve.
Plain English
The longest line you can draw across an oval, going right through the middle from one end to the other.
Context Anchor
Seen in aviation diagrams, navigation discussions, and equipment descriptions when an oval shape or oval area is being described precisely.
Derivation
From Latin major meaning 'greater' and axis meaning 'line about which something turns or is measured.' Together: the greater of the two reference lines of an ellipse, distinguishing it from the shorter minor axis.
Why Pilots Care
If an elliptical area is used to show a path, coverage area, or area of uncertainty, the major axis tells you the longest direction of that oval area.
Analogy
Picture a running track shaped like an oval. The major axis is the long straight line you'd draw down the length of the track, end to end, through the center.
Intuition Check
Do not read “axis” here as one of an airplane’s axes of movement. Here it means a center line drawn through a geometric shape.
Example Sentence 1
The major axis of the ellipse measured twice the length of the minor axis.
Example Sentence 2
Designers align the major axis of an elliptical wing planform with the aircraft span to improve lift distribution.