Plain English Meaning
The pitch axis is the side-to-side line through the airplane that the nose pivots up and down around. Pulling back on the yoke raises the nose; pushing forward lowers it.
Full Definition
The pitch axis, also called the lateral axis, is the imaginary line running through the airplane from wingtip to wingtip, passing through the center of gravity. Rotation around this axis is called pitch, and it is what raises or lowers the airplane's nose. Pitch is controlled by the elevator (or stabilator), which is operated by fore-and-aft movement of the control yoke or stick.
Context Anchor
You see this term when learning the three airplane axes of movement and how the elevator changes the airplane’s or .