A
DNA sequence which is its own
reverse complement. That is, it reads the same forwards and backwards,
swapping
A and
T, and
C and
G. Thus, it's not really a
palindrome. Very many
restriction sites are of this form; it appears extensively in
nature, and even more in the techniques of
molecular biology.
The term is also used for near-palindromes, and even for sequences with beginning and end that are reverse complements. This form is even more prevalent. For instance, most transposons have it.
While not everything about why these palindromes appear is known, there do appear to be sound engineering reasons for it!