Wiktionary
single point of failure
n. a component in a device, or a point in a network, that, if it were to fail would cause the entire device or network to fail; normally eliminated by adding redundancy
Wikipedia
Single point of failure
A single point of failure (SPOF) is a part of a system that, if it fails, will stop the entire system from working. SPOFs are undesirable in any system with a goal of high availability or reliability, be it a business practice, software application, or other industrial system.