Plain English Meaning
A picture that shows how high you are versus how fast you are going, so you can see at a glance how much energy the airplane has and how that energy can shift between height and speed.
Full Definition
A graphical diagram that plots an airplane's altitude on one axis and airspeed on the other, used to visualize the aircraft's current energy state and the trade-offs between potential energy (altitude) and kinetic energy (airspeed) available during flight.
Context Anchor
Seen in energy-management discussions in the Airplane Flying Handbook, especially when visualizing how an airplane changes between different combinations of altitude and airspeed.