The word palindrome is derived from the Greek palÃndromos, meaning running back again (palÃn = AGAIN + drom–, drameîn = RUN). A palindrome is a word or phrase which reads the same in both directions. Some simple examples are:
RACECAR DEED LEVEL PIP
ROTOR CIVIC POP MADAM
EYE NUN RADAR TOOT
Words like LIVE and STRAW (which read EVIL and WARTS backwards) are not themselves palindromes but the "phrases" LIVE EVIL and STRAW WARTS are. A palindrome is not necessarily a single word.
The longest single English word in common usage which is a palindrome is REDIVIDER, although the contrived chemical term DETARTRATED is two letters longer. In Finnish there is a 25-letter palindromic word: SOLUTOMAATTIMITTAAMOTULOS which means the result from a measurement laboratory for tomatoes, although technically it is a compound of four words. There is also the equally long SAIPPUAKUPPINIPPUKAUPPIAS which means soap cup trader.
When creating reversible sentences, it is usually accepted that punctuation and word spacings are ignored, and so the famous MADAM, I'M ADAM is a valid palindrome.
Palindrome Varieties
There are two other types of palindrome, although neither is particularly common. The first is the word-unit palindrome. As the name suggests, these are palindromes in which the words form the same sentence in either direction, e.g. WOMEN UNDERSTAND MEN; FEW MEN UNDERSTAND WOMEN. Occasionally these are also traditional palindromes, such as in I DID, DID I? but this only happens when each word is itself a traditional palindrome.
The other kind of palindrome is the mirrored palindrome. These are palindromes which are graphically reversible. Not all letters in mirrored palindromes necessarily have symmetry about a vertical axis (A, H, I, M, O, T, U, V, W, X) since some letters are (more or less) mirror images of others (e.g. L and J). Therefore a mirrored palindrome is not necessarily also a traditional palindrome. Punctuation and spaces cannot be ignored when creating mirrored palindromes. When reversed A TOYOTA becomes ATOYOT A which, strictly speaking, is not the same. WOT TOW, although fairly meaningless, is a valid mirrored palindrome. Of course, with this type of palindrome, it is important whether upper or lower case letters are used: bid is a mirrored palindrome, but BID is not.
World's Longest Palindrome Sentence? 17,826 words
A 17,826 Word Palindrome (or Palindromic Sentence)
A man, a plan, a cameo, Zena, Bird, Mocha, Prowel, a rave,
Uganda, Wait, a lobola, Argo, Goto, Koser, Ihab, Udall, a revocation,
Ebarta, Muscat, eyes, Rehm, a cession, Udella, E-boat, OAS, a mirage,
IPBM, a caress, Etam, FCA, a mica, Ojai, Lebowa, Yaeger, a barge, Rab,
Alsatian, a mod, Adv, a rps, Ileane, Valeta, Grenada, Hetty, Fayme,
REME, HCM, Tsan, Owena, Tamar, Yompur, Isa, Nil, Lorrin, Riksdag,
Mona, Ronn, O'Conner, Kirk, an okay, Nafl, Lira, Robi, Rame, FIFA, a
need, Rodi, Muharram, Ober, a coma, carri, Hwang, Taos, Salado, Olfe,
Camag, Kdar, a hdkf, Jemina, Nadler, Ehud, Rutan, a baster, Ebn,
aedegi, a gals, Ira, Tepper, a minim, a gowd, Ulda, Ogawa, TSgt,
Callida, Rodl, Ewart, seraphs, Ain, Newgate, Vaden, navettes, Sabah,
Swat, Luci, Pam, Arda, pools, a boar, Akira, Gally, CSC, Avalon, a
tuba, AAM, Artima, AFB, Selah, wellies, Ronald, BArch, nullos, Uni, an
aet, Nadabus, Tyree, Poop, Sennar, CAB, a nanny, Let, Efahan, Dasya,
Moon, Ikaria, Nam, Lamar, Egor, Rover, Tanana, Loki, MIP, PharD,
endia, rates, Punan, Eba, Alva, Paine, BEF, Fagan, nugae, taws, Una,
Woll, a tab, CSE, Kamerad, YCL, a melt, Diehl, Lewellen, Sacci, Reggi,
RFA, BSA, naoi, Kuyp, Oceanic, Sipple, Yalu, Kosey, nota, talers,
Frida, a wab, Musset, Aoede, Erick, a reign, Attica, Marge, Leta, Mat,
Naboth, Saphra, Gila, Arany, Costa, Fasta, Mabel, Udine, Derte,
Medill, Erotes, RuPaul, Uzzi, Waler, Omak, a kaif, Freed, a doc, a
marga, Nilla, Dole, USPO, Ogata, tubas, somata, Dash, Danika, Salida,
Fiji, Emili, Kazak, Oder, CAC, Ilocanos, Nudd, Uda, Susette, Diskin,
Numitor, Munn, a tsarina, Valle, Soraya, Masan, Ezaria, Teri, Luks,
UNDRO, Naida, Wal, a major, Stelu, OGO, Dilan, Iapetus, an areg, a
jacal, Nessi, R. Bayer, Wotan, a tip, Tabb, a stonk, a habu, Salli,
Major, Berosus, a tepal, Layla, Gitana, Cold, Ogun, Sadie, Bowen,
Rock, Cila, Wey, a halo, Omari, Albniz, Osana, Illimani, Laura, Mitzi,
Dacey, a grill, Enyo, Leno, Snashall, a moot, an amt, a tike, Lamaism,
Ygerne, Gainer, ilia, Vala, Alia, Keli, Codie, Bikini, Taliesin, Eyde,
Baiel, DAgr, Obara, Caracas, Niven, Airlie, Vadim, Seda, a rata, Utas,
Olathe, Pohai, Tatum, obli, Nadab, a nook, Corabel, a cercus, Levona,
Sida, Sapir, Otilia, MSIE, Mbm, ILS, Opis, Sudra, Lea, Grieg, a
robalo, Pack, an amp, Otaru, Savil, Spoor, Tarapoto, Kadner, Everard,
Fleur, Nita, Palos, origamis, Lina, Naoma, Lananna, Erdei, trymata,
Paza, Mano, Musa, Toy, Raff, U-boat, Neiman, Illampu, Osanna, Dinah,
Pittel, a valor, Itonia, Tamra, Kamal, Edana, Cly, Redd, a t