What do you call a word that is another word spelled backwards?

One common term is semordnilap (the Word palindrome spelled backwards).

Palindromes, like radar and racecar, spell the same Word forward or backward. Semordnilaps are different real Words when spelled backwards, such as was/saw, liar/rail, diaper/repaid, and desserts/stressed.

Other terms for these Words are opisograms, or reverse anagrams.

(see related link)