sphere is on the Academic Word List.

Pronunciation change

Noun change

Singular
sphere

Plural
spheres

 
A sphere
  1. (countable) A sphere is a shape that is round in all directions, like a ball. Every point on a sphere is the same distance from the centre (a point in the middle) of the sphere.
    The Earth is almost exactly a sphere.
    A drop of water can be a sphere while it's falling.