Learner's definition of MISSING
1 : unable to be found: not in a usual or expected place — often + from— sometimes used in the phrase go missing
◊ This phrase is more common in British English than in U.S. English.
2 : needed or expected but not included
— often + from
The new director has provided the missing ingredient that was needed for the show's success.
It's a good sauce, but there's something missing. [=it lacks something that would make it better]
missing in action
— used to say that a soldier cannot be found after a battle and might have been killed, captured, or wounded