Euclidean geometry is the study of shapes, sizes, and positions based on the principles and assumptions stated by Greek Mathematician Euclid of Alexandria. It is also called the geometry of flat surfaces. Euclidean geometry is limited to the study of straight lines and objects usually in a 2d space. In practice, Euclidean geometry cannot be applied to curved spaces and curved lines.
Euclid’s geometrical mathematics works under set postulates (called axioms). Euclid’s axioms were considered the base of mathematics related to geometry for almost 2000 years until Einstein’s theory of relativity offset his theories and proved his postulates wrong in relative dimensions of geometry.
The entire bulk of Euclidean geometry is based on 5 axioms or assumptions:

A straight line joins two points

A straight line can be elongated on both sides to infinity

A circle can be made if a point is given as the center and a measure is given for its radius

All right angles are equal

If a straight line intersects two lines the interior angles on the same sides will always have a sum less than 90∘.
Any system that violates the said axioms comprises nonEuclidean geometry.