What is an axis of symmetry?

How many axes of symmetry can you find on letter A?
How many axes of symmetry can you find on letter B?
How many axes of symmetry can you find on letter C?
How many axes of symmetry can you find on letter D?
How many axes of symmetry can you find on letter E?
How many axes of symmetry can you find on letter F?
How many axes of symmetry can you find on letter G?
How many axes of symmetry can you find on letter H?
How many axes of symmetry can you find on letter I?
How many axes of symmetry can you find on letter J?
How many axes of symmetry can you find on letter K?
How many axes of symmetry can you find on letter L?
How many axes of symmetry can you find on letter M?
How many axes of symmetry can you find on letter N?
How many axes of symmetry can you find on letter Ñ?
How many axes of symmetry can you find on letter O?