A “stemcell” is a customized operating system image containing the filesystem for BOSH-managed virtual machines.
Stemcells provide a powerful separation between the OS and the other software packages bundled in a deployment. Each stemcell - no matter the underlying infrastructure - is exactly the same. This allows for rapid, reliable mobility between different infrastructure targets. Stemcells are key to the application portability delivered by Pivotal Platform.
Similarly, Pivotal regularly publishes tiles for Pivotal Platform tested against the latest stemcells to remedy identified vulnerabilities.