What is the difference between fitnah (trial) and nazilah (calamity)?

Linguistically, fitnah means tribulation and test. Its origin comes from the melting of silver and gold to distinguish the good from the bad.

In the Quran, fitnah has been used to refer to: Polytheism and disbelief, as in His Almighty's saying: "And fight them until there is no [more] fitnah." Burning with fire, as in His Almighty's saying: "Indeed, those who have persecuted the believing men and believing women." Examination and test, as in His Almighty's saying: "Do people think that they will be left alone because they say, 'We believe,' and will not be tested?"

An-Nazilah is: an important or severe incident that befalls a person. Its plural is an-Nawazil. Among jurists, it refers to new practical jurisprudential issues for which there is no [explicit] text.