Definition
The vertical speed, expressed in feet per minute, that produces the greatest gain in altitude per unit of time for a given aircraft and set of conditions. It is achieved by flying at the airspeed published as best rate of climb (Vy) with climb power set, and is shown on the vertical speed indicator (VSI) once the climb is stabilized.
Plain English
The climb rate that gets you to a higher altitude in the shortest time. You reach it by flying at the airplane's best-rate-of-climb speed with climb power applied.
Context Anchor
Seen when using the vertical speed indicator to judge whether the airplane is climbing at the expected or best practical rate.
Derivation
Optimum comes from the Latin optimus, meaning 'best.' In this context it means the climb rate that is best for gaining altitude quickly — not the fastest possible rate, but the rate that is most efficient for time-to-altitude.
Why Pilots Care
It lets the pilot reach a safe or assigned altitude quickly while conserving fuel and maintaining positive aircraft control.
Intuition Check
Optimum does not mean “as steep as possible.” Rate of climb is about altitude gained per minute, not how high the nose looks.
Example Sentence 1
After lift-off she pitched for Vy and trimmed to hold the optimum rate of climb shown on the VSI.
Example Sentence 2
After takeoff the departure procedure called for maintaining the optimum rate of climb until reaching the initial altitude restriction.