A palindrome is a word or phrase that is the same backwards and forwards. The name was coined in the 1600s by English writer Ben Jonson from the Greek words Palin (back) and Dromos (way or direction), although the first known palindrome was a Latin phrase discovered in 79AD: "Sator Arepo Tenet Opera Rotas". This is known as the Sator square, since it can be written out as a five x five square and is the same whether read across the squares or down.
Other examples of palindromic phrases or sentences include: "Drab as a fool, as aloof as a bard." "Egad, an adage!" "Here so long? No loser, eh?" "Live not on evil deed, live not on evil." " Madam, in Eden I'm Adam."
Most people who live in Adelaide, Australia know what palindromes are because there is a suburb whose name is a palindrome: Glenelg (named after a village in Scotland). Other geographic palindromes include Kanakanak (Alaska), Saxet, Texas, Ellemelle (Belgium), Qaanaaq (Greenland), Kinikinik (Alberta), and Anahanahana (Madagascar).
Palindromic company and brand names include Oxo, Omo, Axa, Aviva, Civic, Rotavator, and M&M, but interestingly enough, there are more companies using Palindrome in their names, than there are palindromic brand names. Palindromic names include Anna, Otto, Elle, Bob, Eve, Hannah. And there's also Mom, Mam, Mum, Dad, Pop, and of course, Madam.
Numbers can also be palindromes, such as 345543 or 9096909, and perfect squares such as 121, 484 and 10201. A date can be a palindrome, such as 6-06-06, as can a time: 21:12, or a year: 2002 or 1991 (and we won't be around for the next one: 2112).
The longest palindrome sentence in English is over 17,000 characters long, but it's completely meaningless. You can read it at: http://www.norvig.com/pal1txt.html, but don't expect it to make any sense because it's computer generated gibberish. The longer palindromes are, the less sense they make. The following is probably about the limit: "Are we not drawn onward, we few, drawn onward to a new era."
The longest English palindrome word is "tattarrattat", invented by James Joyce in "Ulysses" to mean a knock on the door. One of the second longest is detartrated, which means having tartrates (a chemical) removed. Another, also with 11 letters is aibohphobia, an invented word, meaning, appropriately enough: fear of palindromes!