This is a post copied over from the SDMBtemp.
In What do you call a word that spells a different word backwards?, they don’t really answer the question. Or maybe I should say the answer they give is a cop-out.
The real answer is that such words are usually called reversals or occasionally inversions, although sometimes people think they’re clever and coin the word semordnilap for them (I’ve seen three or four writers do this, all thinking they are being original).
At any rate, I called them reversals in my Word Ways article on Reversed Placenames and so does Dmitri Borgmann in his book Language on Vacation.