Main Difference
The main difference between Length and Diameter is that the Length is a measured dimension of an object and Diameter is a straight line segment that passes through the center of a circle.

Length
In geometric measurements, length is the most extended dimension of an object. In the International System of Quantities, length is any quantity with dimension distance. In other contexts, length is a measured dimension of an object. Length may be distinguished from height, which is vertical extent, and width or breadth, which are the distance from side to side, measuring across the object at right angles to the length. For example, it is possible to cut a length of wire shorter than the wire’s width. In most systems of measurement, the unit of length is a base unit, from which other units are derived.
Length is a measure of one dimension, whereas area is a measure of two dimensions (length squared) and volume is a measure of three dimensions (length cubed).

Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid for the diameter of a sphere.
In more modern usage, the length of a diameter is also called the diameter. In this sense one speaks of the diameter rather than a diameter (which refers to the line itself), because all diameters of a circle or sphere have the same length, this being twice the radius r.
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{displaystyle d=2rquad Rightarrow quad r={frac {d}{2}}.}
For a convex shape in the plane, the diameter is defined to be the largest distance that can be formed between two opposite parallel lines tangent to its boundary, and the width is often defined to be the smallest such distance. Both quantities can be calculated efficiently using rotating calipers. For a curve of constant width such as the Reuleaux triangle, the width and diameter are the same because all such pairs of parallel tangent lines have the same distance.
For an ellipse, the standard terminology is different. A diameter of an ellipse is any chord passing through the center of the ellipse. For example, conjugate diameters have the property that a tangent line to the ellipse at the endpoint of one of them is parallel to the other one. The longest diameter is called the major axis.
The word “diameter” is derived from Greek διάμετρος (diametros), “diameter of a circle”, from διά (dia), “across, through” and μέτρον (metron), “measure”. It is often abbreviated DIA, dia, d, or ⌀.

Length (noun)
The distance measured along the longest dimension of an object.

Length (noun)
duration

Length (noun)
The length of a horse, used to indicate the distance between horses at the end of a race.

Length (noun)
Distance between the two ends of a line segment.

Length (noun)
The distance down the pitch that the ball bounces on its way to the batsman.

Length (noun)
Total extent.
“the length of a book”

Length (noun)
Part of something that is long; a physical piece of something.
“a length of rope”

Length (verb)
To lengthen.

Diameter (noun)
Any straight line between two points on the circumference of a circle that passes through the centre/center of the circle.

Diameter (noun)
The length of such a line.

Diameter (noun)
The maximum distance between any two points in a metric space

Diameter (noun)
The maximum eccentricity over all vertices in a graph.

Length (noun)
the measurement or extent of something from end to end; the greater of two or the greatest of three dimensions of an object
“the fish reaches a length of 10 inches”
“the delta is twenty kilometres in length”

Length (noun)
the quality of being long
“the length of the waiting list”

Length (noun)
the length of a swimming pool as a measure of the distance swum
“fifty lengths of the pool”

Length (noun)
the length of a horse, boat, etc., as a measure of the lead in a race
“the mare won the race by seven lengths”

Length (noun)
the extent of a garment in a vertical direction when worn
“the length of her skirt”

Length (noun)
the full distance that a thing extends for
“the muscles running the length of my spine”

Length (noun)
the full extent of one’s body
“he awkwardly lowered his length into the small car”

Length (noun)
the metrical quantity or duration of a vowel or syllable
“the length of the syllable is isomorphic with the length of the syllabic vowel”

Length (noun)
the amount of time occupied by something
“delivery must be within a reasonable length of time”

Length (noun)
a piece or stretch of something
“the surviving length of track”
“a length of brown satin”

Length (noun)
an extreme to which a course of action is taken
“they go to great lengths to avoid the press”

Length (noun)
the distance from the batsman at which a wellbowled ball pitches
“Lewis tended to bowl short of a length”

Length (noun)
(in bridge or whist) the number of cards of a suit held in one’s hand, especially when five or more
“to open one heart with equal length in hearts and spades is in the modern Acol style”