|Version of Microsoft Windows|
|OS family||Windows NT|
|Version||5.5 (version in many files in build 5111)|
5.0 (kernel reports)
Neptune was the codename for an unreleased version of Microsoft Windows. It identifies as NT 5.5 (although many files in build 5111 have the version "5.50.5111.1", and this is even the true version in the kernel, the kernel actually reports "5.00.5111.1" to match the subsystem version in its own PE header), 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 2021, 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 March 2001 presumably to bridge the gap between Neptune and Odyssey.