Definition
The straight-line distance from one edge of a circle to the opposite edge, passing through the center. It is equal to twice the radius and represents the widest measurement across a circular object.
Plain English
The distance straight across a circle, measured through its middle.
Context Anchor
Seen in maintenance instructions and part measurements for round items such as bolts, holes, tubes, shafts, wheels, and seals.
Derivation
From the Greek 'diametros,' meaning 'measure across' (dia = across, metron = measure). The original meaning still fits exactly: it is the measurement straight across a circle.
Why Pilots Care
Many maintenance specifications and tolerances are given as diameters. Reading a diameter as a radius (or vice versa) doubles or halves the value and can lead to selecting the wrong part or mis-measuring wear limits.
Intuition Check
Diameter is not the distance around the outside edge of a circle. It is the straight distance across the center.
Example Sentence 1
The technician measured the diameter of the cylinder bore to check it against the manufacturer's wear limits.
Example Sentence 2
A larger wheel diameter can change the aircraft's ground clearance and takeoff performance.