Definition:Diameter of Ellipse/Also defined as

Definition

Some sources define the diameter of an ellipse $\KK$ as a chord of $\KK$ which passes through the center of $\KK$.

This is a perfectly adequate definition of a diameter of an ellipse.

However, the definition does not work so well in the context of:

a hyperbola, as it does not encompass diameters which are not chords

a parabola, which does not have a center.

Also see

• Locus of Midpoints of Parallel Chords of Central Conic passes through Center

• Results about diameters of conic sections can be found here.

Sources

• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): ellipse
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): ellipse