Definition
Two of the three imaginary reference lines passing through an airplane's center of gravity. The longitudinal axis runs from the nose to the tail; rotation about it is roll, controlled by the ailerons. The lateral axis runs from wingtip to wingtip; rotation about it is pitch, controlled by the elevator.
Plain English
Two invisible lines through the middle of the airplane. One runs nose-to-tail — the airplane rolls (banks) around it. The other runs wingtip-to-wingtip — the airplane pitches (nose up or nose down) around it.
Context Anchor
Seen when learning attitude instrument flying, especially when matching control inputs to what the attitude display shows.
Derivation
Longitudinal comes from the Latin longitudo, meaning length — so the axis along the airplane's length. Lateral comes from the Latin latus, meaning side — the axis running side to side. The names describe exactly which direction each line points through the aircraft.
Why Pilots Care
Pitch and bank — the two attitude changes a pilot manages constantly — happen around these two axes. Understanding them makes attitude instrument flying easier to picture and easier to control precisely.
Analogy
Picture a model airplane with a pencil through its nose and tail, and another pencil through both wings. The airplane can rotate around each pencil in a different way.
Intuition Check
Do not think of these axes as lines on a map or as the airplane’s path through the air. They are imaginary lines through the airplane itself, used to describe how the airplane rotates.
Example Sentence 1
Rolling into a 20-degree bank rotates the airplane about its longitudinal axis.
Example Sentence 2
Elevator input pitches the nose up or down by rotating the airplane around its lateral axis.