Definition
A ring-shaped space or component, typically the area between two concentric circles or cylinders. In aircraft powerplants, the annulus is commonly the ring-shaped passage formed between an inner and outer cylindrical surface, such as the space between the inner and outer casings of a turbine engine combustion chamber or bearing assembly.
Plain English
A doughnut-shaped space — the gap you get between a smaller circle sitting inside a larger one.
Context Anchor
Seen in powerplant maintenance descriptions of engine cases, seals, ducts, and airflow or oil passages.
Derivation
From the Latin annulus, meaning 'little ring' (a diminutive of anus, meaning 'ring'). The word keeps that original sense — a ring-shaped region or part — which is exactly how it is used in engine construction.
Why Pilots Care
A pilot may see this term in maintenance or engine-system discussions. Understanding it helps make sense of descriptions of where air, oil, or engine parts are located around a central opening or shaft.
Analogy
Picture a doughnut viewed from above: the dough itself is the annulus — the ring of material between the hole in the middle and the outer edge.
Intuition Check
An annulus is not just any circle. It means a ring shape or ring-shaped space around a center.
Example Sentence 1
Cooling air flows through the annulus between the combustion liner and the outer casing to keep the liner from overheating.
Example Sentence 2
Erosion in the turbine annulus reduces engine efficiency and thrust.