Plain English Meaning
The speed that gets you the most height in the shortest distance over the ground. It is the steepest climb the airplane can fly.
Full Definition
The airspeed (V_X) that produces the greatest gain in altitude per unit of horizontal distance traveled. It delivers the steepest climb path, used when terrain or obstacles must be cleared shortly after takeoff.
Context Anchor
You see this term in takeoff planning, obstacle-clearance practice, and the airplane’s operating handbook climb-speed information.
Derivation
"Angle" here refers to the climb path's slope relative to the ground. "Best angle" means the steepest slope the airplane can sustain — the most altitude gained for the least ground covered.