Geometry is the branch of mathematics that deals with the study of shapes, sizes, and properties of figures and spaces. (Mathematics is the study of numbers and shapes. Figures are shapes like squares, circles, and triangles. Spaces are areas or volumes.)
For example, when you measure the length of a pencil or draw a square, you are using geometry.
Very Simple Definition:
Geometry is about studying shapes and how they are different from each other. When you draw a circle or a triangle, you are doing geometry.