There are two different threads reporting issues with Google's define operator.
But people using the [define:word] command in Google are being taken to a error page that reads:
A search for [define:apple] now return:
No definitions were found for apple.
- Make sure all words are spelled correctly.
- Search the Web for documents that contain "apple"
Like I said, I tried it myself and I get Google Experimental logo and taken directly to Google's definitions page.
It may be that Google is testing and rolling out a new interface for definitions soon. Google Operating System has a screen shot of the new interface, which embeds the definitions interface within the normal Google search results user interface. Here is a picture:
I do suspect it is a bug in the user interface test, which is why I can't reproduce it.