Windows as a service
Windows as a service is the development model Microsoft has adopted for Microsoft Windows after the release of Windows 10. Instead of releasing a major version of Windows every few years, the company instead releases smaller feature updates in shorter intervals.
Since Windows transitioned to Azure's semester model in 2019, new features are planned with respect to the so-called semesters rather than feature updates. From a development viewpoint, semesters are equivalent to releases as the active development branch goes through feature development, stabilization and sign-off phases during each semester. However, they are independent from the actual releases available to the public as the company does not necessarily release a new version at the end of each semester, e.g. three semesters passed between Windows 11 and Windows 10 May 2020 Update (Manganese, Iron, Cobalt).
Recent rumors suggest that Microsoft might be moving back to a traditional release schedule, with a major version of Windows releasing every three years. The period between two major versions would then be spanned by shipping minor feature updates called Moments, up to four times a year.