|Version of Microsoft Windows|
|OS family||Windows NT|
Neptune was the codename for an unreleased version of Windows. It identifies as NT 5.5, and was intended to be the first consumer version of Windows NT. The project, however, was scrapped together with Odyssey, which was the planned next major version that would succeed Neptune, and replaced with Whistler. As of 2020, only one build has leaked, but others may exist.
According to antitrust documents, a minor update to Neptune called Triton was planned to be released 12 months after the initial release of Neptune in 2001-03 presumably to bridge the gap between Neptune and Odyssey.