Definition
In a level turn, the single combined force produced by adding the airplane's weight (acting straight down) and the centrifugal force (acting horizontally outward from the turn). The lift produced by the wings must equal and oppose this combined force for the airplane to maintain altitude in the turn.
Plain English
When an airplane turns, two forces pull on it at the same time: gravity pulling it down, and an outward pull from the curving path. Add those two together and you get one bigger force pulling down and outward. The wings have to lift hard enough, in the right direction, to cancel that combined pull.
Context Anchor
Seen in aerodynamics discussions of turns, especially diagrams showing why a steeper bank requires more lift.
Derivation
Resultant' comes from the Latin resultare, meaning 'to spring back' or 'to follow as a consequence.' In physics it means the single force that results from combining two or more forces. Knowing that helps here: weight and centrifugal force don't disappear when added — they combine into one effective force the lift must overcome.
Why Pilots Care
It determines the load factor and therefore how much extra lift the wings must produce to hold altitude in a bank.
Analogy
Swing a bucket of water on a rope in a circle. Gravity pulls the bucket down, the circular motion pulls it outward, and the rope has to pull against both at once. The rope's pull is doing the same job as the wings' lift in a turn.
Grounding Statement
Picture a level steep turn: you feel heavier in the seat because the airplane must produce more lift to oppose the stronger combined pull.
Intuition Check
Do not think of the resultant as a new, separate force added to the airplane. It is the single combined effect of weight and the outward-feeling centrifugal force during the turn.
Example Sentence 1
In a level turn, lift must equal the resultant of weight and centrifugal force, which is why back pressure is needed to hold altitude.
Example Sentence 2
If lift does not match the resultant of weight and centrifugal force the airplane will either lose or gain altitude during the turn.