  This paper presents a taxonomy that allows defining the fault tolerance
regimes fail-operational, fail-degraded, and fail-safe in the context of
automotive systems. Fault tolerance regimes such as these are widely used in
recent publications related to automated driving, yet without definitions. This
largely holds true for automotive safety standards, too. We show that fault
tolerance regimes defined in scientific publications related to the automotive
domain are partially ambiguous as well as taxonomically unrelated. The
presented taxonomy is based on terminology stemming from ISO 26262 as well as
from systems engineering. It uses four criteria to distinguish fault tolerance
regimes. In addition to fail-operational, fail-degraded, and fail-safe, the
core terminology consists of operational and fail-unsafe. These terms are
supported by definitions of available performance, nominal performance,
functionality, and a concise definition of the safe state. For verification, we
show by means of two examples from the automotive domain that the taxonomy can
be applied to hierarchical systems of different complexity.
