Definition
The angular movement of the elevator, the hinged horizontal control surface at the tail of the airplane, away from its neutral (streamlined) position in response to fore-and-aft movement of the control yoke or stick. Pulling back deflects the elevator upward, which raises the nose; pushing forward deflects it downward, which lowers the nose. The amount of deflection is the size of that angular displacement.
Plain English
How far the small flap at the tail end of the airplane is tilted up or down from its level resting position when the pilot pulls or pushes on the controls. More tilt means more change in the nose's pitch.
Context Anchor
Seen during the normal takeoff roll when the pilot uses back pressure to raise the nose at the proper speed.
Derivation
Elevator' comes from the Latin elevare, 'to lift up,' because this surface is what raises (or lowers) the nose. 'Deflection' comes from the Latin deflectere, 'to bend away from.' Together it means 'how far the lifting surface has been bent away from its neutral position.'
Why Pilots Care
The amount and direction of elevator deflection directly controls rotation speed and pitch attitude on takeoff; too little delays liftoff while too much risks a tail strike or excessive angle of attack.
Intuition Check
“Elevator” here does not mean a lift in a building; it means the tail control surface that helps control the nose. “Deflection” does not mean damage or bouncing off something; it means movement away from the neutral position.
Example Sentence 1
As airspeed built during the takeoff roll, the pilot applied gentle back-pressure, and the resulting elevator deflection raised the nose to the takeoff attitude.
Example Sentence 2
On a short-field takeoff the pilot uses a slightly greater elevator deflection after rotation to establish the best angle of climb attitude.