A palindrome is a word, phrase, sentence, or even a passage which is completely reversible - in other words, if it's letters are written in reverse, they spell exactly the same thing (punctuation and spacing does not need to be included).

Single word examples include "madam" and "radar", but palindromes can be of any length, and there are many sentence- and even paragraph-length palindromes. The longest I have come across is over three hundred words long, and yes - it's completely reversible.