A palindrome is a word, phrase, number or other sequence of units that has the property of reading the same in either direction (the adjustment of punctuation and spaces between words is generally permitted).

Composing literature in palindromes is an example of constrained writing. The word "palindrome" was coined from Greek roots palin (πάλιν; "back") and dromos (δρóμος; "way, direction") by English writer Ben Jonson in the 1600s.

The actual Greek phrase to describe the phenomenon is karkinikê epigrafê (καρκινική επιγραφή; crab inscription), or simply karkiniêoi (καρκινιήοι; crabs), alluding to the backward movement of crabs, like an inscription which can be read backwards.

Characters

The most familiar palindromes, in English at least, are character-by-character: the written characters read the same backwards as forwards. Palindromes may consist of a single word (civic, level, racecar, Malayalam), or a phrase or sentence ("Neil, a trap! Sid is part alien!", "Was it a rat I saw?", "Mr. Owl ate my metal worm", "Sit on a potato pan, Otis", "Satan, oscillate my metallic sonatas", "Rats live on no evil star."). Spaces, punctuation and case are usually ignored.

Three famous English palindromes are "Able was I ere I saw Elba" (which is also palindromic with respect to spacing), "A man, a plan, a canal-Panama!", and "Madam, in Eden I'm Adam,". The last example is still palindromic if "in Eden" is omitted; the response is either the one-word palindrome, "Eve," or the more obscure "Name no one man."

Some individuals have names that are palindromes. Some changed their name in order to be a palindrome (one example is actor Robert Trebor), while others were given a palindromic name at birth (such as Neo-Nazi philologist Revilo Oliver). Fictional names include Stanley Yelnats, the main protagonist in the book Holes by Louis Sachar.

Words

Some palindromes use words as units rather than letters. Examples are "fall leaves after leaves fall", "First Ladies rule the State and state the rule: ladies first" and "Girl, bathing on Bikini, eyeing boy, sees boy eyeing bikini on bathing girl". The command "Level, madam, level!", composed only of words that are themselves palindromes, is both a character-by-character and a word-by word palindrome.

Lines

Still other palindromes take the line as the unit. The poem Doppelganger, composed by James A. Lindon, is an example.

The dialogue "Crab Canon" in Douglas Hofstadter's Gödel, Escher, Bach is nearly a line-by-line palindrome. The second half of the dialog consists, with some very minor changes, of the same lines as the first half, but in reverse order and spoken by the opposite characters (i.e., lines spoken by Achilles in the first half are spoken by the Tortoise in the second, and vice versa). In the middle is a non-symmetrical line spoken by the Crab, who enters and spouts some nonsense, apparently triggering the reversal. The structure is modeled after the musical form known as crab canon, in particular the canon a 2 cancrizans of Johann Sebastian Bach's The Musical Offering.

Numbers

A palindromic number is a number where the digits, with decimal representation usually assumed, are the same read backwards, for example, 58285. They are studied in recreational mathematics where palindromic numbers with special properties are sought. A palindromic prime is a palindromic number that is a prime number.

Dates

Palindromic dates are of interest to recreational mathematicians and numerologists, and sometimes generate comment in the general media. Whether or not a date is palindromic depends on the style in which it is written. For example, in the dd/mm/yyyy style, the 20th February 2002 (20-02-2002) was palindromic.

_{Much longer palindromic compositions have been constructed and also there are musical and computer program palindromes available these days; follow the below link for more info about Palindromes.}