Congruent: The same

Area: Space inside a 2D object

Perimeter: Length around the outside of a 2D object

Pi (𝝅) : The ratio of a circle's circumference to its diameter.

Perpendicular: At an angle of 90 degree to a given surface

Formula: A mathematical relationship / rule given in symbols. 

Infinity: A number without a given ending (too great to count to the end of the number) - never ends

Sector: A part of the circle enclosed by two radii and an arc.