Superscript two
On Windows, hold Alt and type 0178 or 253 on the number pad, then let go of Alt.
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About the superscript two
In m² or x², the raised two means squared. Latin-1 found room for just three raised digits, ¹, ² and ³, so these are the ones keyboards and fonts handle most easily; the other superscript digits live in a separate Unicode block.