Any device or communications line that has no redundancy is a single point of failure (SPOF). If it fails, the system stops. Every non-redundant component in a system is an SPOF. See
fault tolerant and
RAS.
21st Century - Countless SPOFs
People rarely have an extra wireless router on hand at home. If the router fails: no Internet. A website hosted by only one Web server is a single point of failure, and there are millions of websites using only one server. There are myriad more single points of failure than there are redundant components and failsafe systems running today. Fortunately, most modern electronics work for many years.