Webster's Third New International Dictionary defines palindrome as "a word, verse, or sentence that reads the same backward as forward". As is often the case with limericks, sonnets, and other highly-structured but nowadays seldom-pursued literary creations, the few frequently-anthologized examples have come to be the only examples we know. This is certainly the case with the palindrome.
Chism, Steven J.
"On Teaching Palindromes,"
Word Ways: Vol. 25
, Article 4.
Available at: http://digitalcommons.butler.edu/wordways/vol25/iss4/4