Question

What is it called when letters form a word both forward and backward, like STOP and POTS (not a palindrome)?

Answer 1

The most common term for this (currently) is semordnilap

Explanation:

Although it does not appear in (at least my copy of) the Oxford Dictionary, "semordnilap" ("palindromes" spelled backward) is the term that is most often applied for this. (many other terms have been suggested over the years).