A fault-tolerant computer, network, or machine is built so it can keep working when something goes wrong — a part fails, a disk dies, a program hits an error. The fault is still there; the system is designed not to fall over because of it. Engineers talk this way about servers, planes’ computers, and power systems: extra parts, copies of data, ways to switch to a backup. It does not mean the machine never has faults, and it does not mean the fault is someone else’s blame. Though actually, everyday English fault often means “who did wrong”; here it means “a breakdown.” The hyphenated adjective is at home in technical talk. You will see it on spec sheets more than in kitchen chat. People also say the noun fault tolerance for the same idea.