New word for me. I encountered it for the first time in last Saturday's New York Times crossword puzzle.
21D. This is a debut entry, although its singular form appeared once before. “Words that form other words when read backward” are SEMORDNILAPS, which have a kinship to palindromes, which read the same forward and backward, of course. As you might have noticed, the singular form, “semordnilap,” is palindromes spelled backward. Martin Gardner, a prolific writer and magician, coined the term.