bottom turtle

In contrast to "turtles all the way down", an expression of the problem of infinite regress, the "bottom turtle" is the foundation upon which everything else is built.

An example from computer security:
You have some software with a secret that you need to protect, so you encrypt it. Now you have a key, which is really another kind of secret that you need to protect, so you encrypt it, and so it becomes turtles all the way down. To break this loop, we need to establish a bottom turtle. There must be a secure root of trust either in hardware or a trusted external system upon which the security of the system is built.

To break the loop of infinite regress we need to establish the bottom turtle.