Is there a term for words that when reversed, form other words?

Semordnilap is a common name for them, which has been pretty well accepted.

I know that Carroll used them in his work, but I think the term (read it backwards) was invented later.

Wikipedia's Palindrome article gives semordnilap, as Mark Wallace's answer correctly pointed out. It also lists several alternatives:

volvograms, heteropalindromes, semi-palindromes, half-palindromes, reversgrams, mynoretehs, reversible anagrams, word reversals, or anadromes.

Such pairs could also reasonably be called mutual palindromes, by analogy with mutual recursion and mutual quine

It is a type of anagram:

An anagram is a type of word play, the result of rearranging the letters of a word or phrase to produce a new word or phrase, using all the original letters exactly once; e.g., orchestra = carthorse,

Any word or phrase that exactly reproduces the letters in another order is an anagram.

Nota Bene: The emphasis is mine.