inotify is a Linux kernel subsystem that acts to extend filesystems to notice changes to the filesystem, and report those changes to applications. It replaces an earlier facility, dnotify, which had similar goals.
The original developers of inotify were John McCutchan, Robert Love and Amy Griffis. It has been included in the mainline Linux kernel from release 2.6.13 (June 18, 2005), and could be compiled into 2.6.12 and possibly earlier releases by use of a patch.
One major use is in desktop search utilities like Beagle, where its functionality permits reindexing of changed files without scanning the filesystem for changes every few minutes, which would be very inefficient. By being told that a file has changed directly by the kernel, rather than actively looking, Beagle and such utilities can achieve change-to-reindexing times of only about a second.