What is Palindrome?
References & Definitions

1. The Compact Edition of the Oxford English Dictionary p.258
Palindrome [ad. Gr. running back again] a. sb. A word, verse or sentence that reads the same when the letters composing it are taken in the reverse order. b. adj. That reads the same backwards as forwards. 1706 PHILLIPS, Palindrome .. as Lewd did I live, and evil I did dwel. 1821 New Monthly Mag. II.
170 The Palindromes, or Canorine, or recurrent verses, as they were called. Ibid. 171. In English but one Palindrome line is known.

2. The Imperial Dictionary of the English Language Vol. V p.358
Palindrome (pal'in-dr”m), n. [Gr. palindromos running back - palin,
again, and dromos, a running.] A word, verse, or sentence that is the
same when read backward or forward. The English language has few
palindromes; one represents our first parent politely introducing himself to Eve thus: 'Madam, I'm Adam.' Another (by Taylor, the Water Poet) is 'Lewd did I live, and evil I did dwel.'

3. The New Collins Concise English Dictionary 1984 p.813
n. a word or phrase the letters of which, when taken in reverse order,
read the same: able was I ere I saw Elba. 
[C17:< Gk. palindromos running back again]

4. The Reader's Encyclopedia Second Edition 1965 p.752
palindrome (Gr. palin dromo , to run back again). A word or line the
same backward and forward. Examples are Madam, I'm Adam, also
Roma tibi subito motibus ibit amor. They have also been called Sodat
ics, from their reputedly inventor, Sodates, a scurrilous Greek poet of the 3rd century B.C.

Probably one of the longest palindromes in English is:
"Named undenominationally rebel, I rile Beryl? La, No! I tan. I'm, O Ned, nude, man!"
Try it - it works!
named undenominationally rebel i rile beryl la no i tan im o ned nude man.

More references to Palindromes can be found in Ted Clarke's new book "HOW THICK IS A SHADOW?"