Definition
The brief interval during the entry to a constant-airspeed climb in which the aircraft slows from cruise airspeed to the chosen climb airspeed after the pitch attitude has been raised and climb power has been set.
Plain English
The short stretch of time at the start of a climb where the airplane is slowing down to its climb speed before settling into a steady climb.
Context Anchor
Seen in instrument flying when transitioning from level flight into a straight climb and watching the airspeed settle toward the climb speed.
Derivation
From Latin 'de-' (away from) and 'celer' (swift), meaning to reduce speed. The phrase simply names the window of time during which that slowing takes place.
Why Pilots Care
During this period the pilot must hold the new pitch attitude and let the airspeed bleed off smoothly to the target climb speed. Chasing the airspeed too aggressively here causes pitch oscillations and altitude excursions, both of which matter on instruments.
Grounding Statement
Picture raising the nose for a climb: the airplane starts upward, but its speed bleeds off for a short time before it settles at the climb speed.
Intuition Check
Do not read deceleration period as braking or simply reducing power. Here it means the brief slowing that happens as the airplane transitions into a climb.
Example Sentence 1
After raising the nose and setting climb power, the pilot held the new attitude through the deceleration period until the airspeed stabilized at the climb speed.
Example Sentence 2
Recognizing the end of the deceleration period lets the pilot fine-tune power without overcontrolling airspeed.