A word like OTTO can be spelled the same both forward and backwards but the word NOON (in capitals) can also be spelled upside down. Is there a separate grammar descriptive word for this unusual feature?

Palindrome

Words that can be read upside-down are called ambigrams.

Wow, I never knew that, Sara. I'll have to tell my friend that one!

Thanks Ren33. I knew palindrome but didn't know ambigram. Is ambigram considered a subset of palindrome?

Well it seems to apply more to words which are written upside down and read the same, not really palindromes. I will try and find an example as it is hard to explain.
Later... ah here you go. If you run the mouse over the words they invert.
You can google 'ambigram ' and find lots
I am going to try and find ambigram palindromes now...

Hmm, most of the ones at that link only work because they are written funny..... I suppose the pool of these words that would work without the fancy lettering is pretty darn small. All I can think of is OHO, which isn't really a word.

Yes, Old English works best I suppose. Some are quite hard to decipher.
Do you know that if you write :
T
A
X
I

it reads OK from the back as well,for instance if it were written as a neon sign. Fascinating subject.

To answer my own question ("Is ambigram considered a subset of palindrome?") -- No, they would be a separate category. A word like NOON would be both a palindrome and ambigram, but a word like MOW would be an ambigram but not a palindrome. Found these two helpful pages:

http://www.usingenglish.com/glossary/palindrome.html

http://www.usingenglish.com/glossary/ambigram.html

Check out John Langdon's site.

http://www.johnlangdon.net/

To my husband's despair, I've spent minutes (according to me) and hours (according to him) perusing his work.

I hadn't visited this site for a while--months at least--and can't say I understand his rising Obama. Looks like the Wizard of Oz.
http://blogs.salon.com/0001705/images/great+powerful_oz.gif

Check out Dan Brown's site http://www.danbrown.com/secrets/ambigram.html

That is why I have to spend time here, you learn something new all the time.:) Well that is my excuse and I'm sticking to it.

It all has to do with the axes of symmetry of the capital letters of our Alphabet. A pleasnt blending of mathematics and Literacy.