Definition
A duct or passage whose internal cross-section first narrows (the convergent section) to a minimum area called the throat, then widens again (the divergent section). In gas turbine engines and supersonic exhaust nozzles, this shape is used to accelerate gas flow to supersonic speeds: the convergent portion accelerates the gas to the speed of sound at the throat, and the divergent portion continues accelerating it beyond the speed of sound while pressure drops.
Plain English
A pipe-like passage that first gets narrower, then gets wider. The narrow point in the middle is called the throat. This shape is used to speed up gas flow to faster than the speed of sound.
Context Anchor
Seen in turbine engine exhaust nozzle and gas-flow discussions, especially where high-speed jet thrust is being explained.
Derivation
Convergent comes from Latin convergere, 'to incline together' — the walls come together. Divergent comes from divergere, 'to incline apart' — the walls move apart. So the name literally describes the shape of the passage: walls coming together, then moving apart.
Why Pilots Care
It produces efficient thrust in high-speed aircraft by accelerating exhaust gases to supersonic velocities.
Analogy
Picture an hourglass-shaped passage for air instead of sand. The flow is squeezed through the narrow middle, then allowed to spread out after it passes through.
Grounding Statement
In operation, the gas is squeezed as the duct narrows, then expands as the duct widens, and that expansion can turn pressure into very high gas speed.
Intuition Check
Do not read this as just a pipe that changes size. In this context, the narrowing-then-widening shape is designed to control gas speed and pressure, not merely to connect two parts.
Example Sentence 1
The afterburning turbojet uses a convergent-divergent duct in its exhaust nozzle to produce supersonic flow at full power.
Example Sentence 2
During high-speed flight the convergent-divergent duct allowed the engine to reach supersonic exhaust speeds.