Main Difference
The main difference between Rhomboid and Rhombus is that the Rhomboid is a parallelogram in which adjacent sides are of unequal lengths and angles are oblique and Rhombus is a quadrilateral in which all sides have the same length

Rhomboid
Traditionally, in twodimensional geometry, a rhomboid is a parallelogram in which adjacent sides are of unequal lengths and angles are nonright angled.
A parallelogram with sides of equal length (equilateral) is a rhombus but not a rhomboid.
A parallelogram with right angled corners is a rectangle but not a rhomboid.
The term rhomboid is now more often used for a rhombohedron or a more general parallelepiped, a solid figure with six faces in which each face is a parallelogram and pairs of opposite faces lie in parallel planes. Some crystals are formed in threedimensional rhomboids. This solid is also sometimes called a rhombic prism. The term occurs frequently in science terminology referring to both its two and threedimensional meaning.

Rhombus
In Euclidean geometry, a rhombus (◊) (plural rhombi or rhombuses) is a simple (nonselfintersecting) quadrilateral whose four sides all have the same length. Another name is equilateral quadrilateral, since equilateral means that all of its sides are equal in length. The rhombus is often called a diamond, after the diamonds suit in playing cards which resembles the projection of an octahedral diamond, or a lozenge, though the former sometimes refers specifically to a rhombus with a 60° angle (see Polyiamond), and the latter sometimes refers specifically to a rhombus with a 45° angle.
Every rhombus is a parallelogram and a kite. A rhombus with right angles is a square.

Rhomboid (noun)
A parallelogram which is neither a rhombus nor a rectangle

Rhomboid (noun)
Any of several muscles that control the shoulders

Rhomboid (noun)
A solid shape which has rhombic faces

Rhomboid (adjective)
resembling, or shaped like a rhombus or rhomboid

Rhombus (noun)
Any of several flatfishes, including the brill and turbot, once considered part of the genus Rhombus, now in ver=170601. from 16th c.

Rhombus (noun)
Snails, now in genus Conus or family Conidae.

Rhombus (noun)
A parallelogram having all sides of equal length. from 16th c.

Rhombus (noun)
a quadrilateral all of whose sides have the same length.