Many sources give the longest single-word palindrome as: tattarrattat.

A palindrome is a word or phrase which reads the same forwards as backwards, i.e. with its 'sides reversed' to create even longer palindromes, just insert "sides reversed is".
So "Able was I, ere I saw Elba' becomes:
Able was I ere I saw Elba sides reversed is able was I ere I saw Elba.