a.k.a. Vanilla Android and AOSP (Android Open Software Project)
Stock Android is the version of Android, which comes without any modifications by a vendor like Fairphone or by Google (no #dic:gapps)
On FP1 Stock Android can be installed in the Advanced Mode of the Fairphone Updater. If you installed Stock Android by accident and want to go back to Fairphone OS, it is recommended to install Fairphone OS manually.