What is PALINDROME ?

The language consisting of Λ (Null String) and the strings s defined over an Alphabet Σ such that
Rev(s)=s.
Some example words of this language are aa
As Rev(aa) = aa
aba
As Rev(aba) = aba
bbb
As Rev(bbb) = bbb
aabaa
As Rev(aabaa) = aabaa
bbbaaabbb
As Rev( bbbaaabbb ) = bbbaaabbb
It is to be noted that the words of PALINDROME are called palindromes.