grem·lin / ˈgremlin/ • n. inf. an imaginary mischievous sprite regarded as responsible for an unexplained problem or fault, esp. a mechanical or electronic one: a gremlin in my computer omitted a line. ∎  such a problem or fault.

gremlin an imaginary mischievous sprite regarded as responsible for an unexplained problem or fault, especially a mechanical or electronic one. From the 1940s, originally as RAF slang, and perhaps suggested by goblin.