Difference Between Diameter Diagonal
A circle is a geometric shape in which the locus of a point on a plane such that the distance of that point is constant from a fixed point on that same plane; this fixed point is called the centre of the circle, and the constant distance from the centre is called the radius of the circle. Similarly, a sphere is a threedimensional closed figure whose surface is a set of points in a space equidistant to a fixed point, called the centre of the sphere.
A polygon is a closed twodimensional curve made of line segments. These line segments are called edges or sides of a polygon, and the common point of two edges is called vertices. For example, triangle, quadrilateral, pentagon etc. A polyhedron is a threedimensional closed figure whose faces are a polygon. Some examples of polyhedrons are cube, cuboid, prism etc. Let us briefly discuss diameter and diagonal as a part of a circle, sphere, polygons and polyhedrons and differentiate between them.
Diameter of a Circle
The chord of a circle is a line segment within the circle that joins any two points on the circle. And hence the longest chord of a circle that passes through the centre of the circle is called the diameter of the circle.

Now that we have seen what diameter is let us briefly discuss what is diagonal before making a differentiation between them.
What is Diagonal?
For any closed curve made of line segments, whether 2dimensional or 3dimensional, a diagonal of a shape is a line segment that joins two vertices of the shape other than the edge of the shape.
Hence, a diagonal is a line segment whose endpoints are two nonadjacent vertices of the figure.

Learn more about what is diagonal.
Difference Between Diameter and Diagonal
Diameter  Diagonal 


Hence, diameter is related to a circle, and all its geometrical properties are related to the properties of a circle. In contrast, a diagonal is associated with a polygon. Despite both being a line segment joining any two points on a geometric figure, they are different.