Plain English Meaning
The speed that gets you the most altitude per minute. Fly at this speed when you want to climb upward as quickly as possible measured by the clock, not by distance over the ground.
Full Definition
The airspeed that produces the greatest gain in altitude per unit of time for a given airplane configuration and weight. It is the speed at which excess power (power available minus power required) is greatest, allowing the airplane to climb the most feet per minute.
Context Anchor
You will see this term in takeoff and climb procedures, especially when changing from obstacle clearance to a .