A palindrome is a word, number, phrase, or other sequence of characters which reads the same backward as forward, such as madam, racecar, or the number 10801. Sentence-length palindromes may be written when allowances are made for adjustments to capital letters, punctuation, and word dividers, such as "A man, a plan, a canal, Panama!", "Was it a car or a cat I saw?" or "No 'x' in Nixon".
Composing literature in palindromes is an example of constrained writing.
The word palindrome was first published by Henry Peacham in his book, The Truth of Our Times (1638). It is derived from the Greek roots palin (πάλιν; "again") and dromos (δρóμος; "way, direction"); however, the Greek language uses a different word, i.e. καρκινικός, to refer to letter-by-letter reversible writing.
Palindromes date back at least to 79 AD, as a palindrome was found as a graffito at Herculaneum, a city buried by ash in that year. This palindrome, called the Sator Square, consists of a sentence written in Latin: "Sator Arepo Tenet Opera Rotas" ("The sower Arepo holds with effort the wheels"). It is remarkable for the fact that the first letters of each word form the first word, the second letters form the second word, and so forth. Hence, it can be arranged into a word square that reads in four different ways: horizontally or vertically from either top left to bottom right or bottom right to top left. As such, they can be referred to as palindromatic.
