Math wasn't just thought about overnight. There were many people and concepts that made the concept of mathematics the idea it is today. I will be listing my personal list of top 5 biggest discoveries or milestones in math history that have been used throughout the timeline of mathematics.
One of my personal five milestones includes the Pythagorean Theorem. The Pythagorean Theorem is defined the relationship between the three sides of a right triangle. It states that the hypotenuse squared is equal to the sum of the squares of the other two sides. This was a milestone because it led to other characteristics of triangles such as SAS, AAS, etc. Thus, another milestone I believe to make math history is the Distance Formula. The Distance Formula is when you are given two points on a coordinate plane and can find the length or the distance between them. This formula can be performed on a coordinate plane or it can be useful to figure out lengths of triangles that are not right triangles.
The next couple milestones I believe to be beneficial to helping create the future discoveries of mathematics. A man by the name of Albert Einstein with the creation of the formula that expresses the relationship between mass and energy (E=mc^2). This led way in both the math department and the science department advances. Another milestone deals with Euclidean Geometry. The acknowledgement of the fifth axiom allowed the creation of many different quadrilaterals. Finally, the development or creation of the various areas and perimeter of multiple shapes includes circle, square, triangle, cone, etc. There are plenty more findings in the mathematic subject that feed off each other which when you think about it is pretty neat fact.