a straight line connecting the center of a circle with two points on its perimeter (or the center of a sphere with two points on its surface)
"The engineer measured the diameter by drawing a straight line from the rim, through the central hub, to the opposite edge of the wheel."
Any straight line between two points on the circumference of a circle that passes through the centre/center of the circle.
"To cut the perfect circular piece of wood, I measured the diameter from edge to edge right through the center knot."
In plain English: Diameter is the straight line that goes all the way across a circle, passing right through its center.
"The diameter of the circular table is exactly two meters."
Usage: Diameter refers to the measurement of a straight line passing through the center of a circle or sphere and connecting two opposite points on its edge. It represents the widest dimension across a circular object, equaling twice the radius.
The word "diameter" comes from the Latin diametros, which was borrowed into Old French before entering English. It originally described a line measuring across something and is built from Greek roots meaning "across" and "measure."