TEE is short for the Trusted Execution Environment and is a trusted OS running in a secure environment, such as TrustZone on ARM CPUs or a separate secure co-processor. We've previously covered the work on TEE for Linux and obviously not everyone is happy about "trusted" computing efforts on Linux and implementations like TEE.
With Linux 4.12 they are hoping to merge TEE via the ARM SoC tree. Jens Wiklander and others believe the subsystem is mature enough where it should now be merged. The TEE subsystem deals with TEE driver registration, managing shared memory between Linux and the TEE, and providing a generic API to the Trusted Execution Environment.
The request for it to be merged for Linux 4.12 was made yesterday on the Linux kernel mailing list.