The line (or surface) across which things are (exactly) the same.
Look in a mirror. Draw a line from your head to between your feet. You are considered symmetrical to your left/right ... but not up/down nor front/back.
The question should have been written as "What are axes of symmetry?" because axes is plural and axis is singular.
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There are infinitely many axes of symmetry in mathematics.
A regular pentagon has five axes of symmetry.
2 axes from angles to angles
4
one