Typical software versioning schemes (e.g. v3.1.45) count ever upwards with each new revision. Kelvin versioning does the opposite, counting down towards absolute zero. When the code in question reaches version 0, it is considered final and frozen, and there will be no further revisions. This system is used when a piece of code is intended to become a permanent fixed standard, that can be depended upon to not change ever.
Kelvin versioning is most notably used for Nock, which is currently at version 4K - four versions away from its final, permanent form. The standard libraries and kernel modules of Arvo are also versioned in this way.