RTM is an acronym for “Release To Manufacturing”. In the software industry, it’s usually used as a label for a certain released version of a piece of software. For example, before Microsoft releases a new version of Windows to the public, they’ll release several test versions (called “beta” versions) to a select few, then a larger release to some developers, then a test release that they believe is done, called a release candidate “RC”. Finally, once they’ve decided that no more changes will be made, they’ll release the product to the manufacturers (RTM) so that they can start mass producing DVDs for sale in stores and distribution with new PCs.
An RTM version isn’t necessarily held back for the DVD producers to ship. Microsoft generally releases the RTM to paid subscribers to their Microsoft Developer Network (MSDN) several months before the official release date. For example, with Windows 7, Microsoft made the RTM version available for download on their MSDN website in early August 2009, even though the same version wasn’t released until October 22, 2009 for the general public to purchase or to receive on a new PC.