Correct spelling, explanation: the word medieval can be translated as of the Middle Ages and comes from the Latin phrase medium aevum. Medieval is actually the combination of the words middle and ev. The former one meant middle while the latter one meant age. Medieval entered English in the 19th century.

Definition of medieval:
adjective – connected with the historical period of the Middle Ages,
The old town has kept its magical, medieval look, which attracts a lot of tourists.

Collocations with medieval:
Some popular collocations with medieval include: medieval architecture, medieval times, and medieval castle.
The place is famous for its medieval architecture as most of the buildings still have their original look.
Medieval times are an important period in history, but we can’t forget it was also extremely cruel.
There is nothing special about the town except a medieval castle that overlooks the area.

Incorrect spelling, explanation: medieval is often misspelled by many English users. It probably results from the fact that the second part of the word is often associated with another English word, which is devil. The consequence of that is the form medevil, which is incorrect. The only right spelling is medieval.

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Incorrect spelling