1. the size or extent of something
the magnitude of the task/achievement
Something of 'magnitude' is big or huge or of significance. "The magnitude of the problem is only just becoming apparent." "We have never witnessed a storm of this magnitude before." This is a noun that can also be used to describe events of importance. "The Queen's visit was of enormous magnitude in terms of the two country's relationship." You are more likely to hear this word used in formal contexts.