What word is the same frontwards backwards and upside down?

Answer 1

Capital letters which read the same normally and upside down are: H, I, N, O, S, X, and Z.

Words that can be made from those to satisfy your criteria might include:

NOON

NON

SIS (slang)

SOS (acronym)

Answer 2

I as in **I** am or **I** do.

Answer 3

NOON