Middle French
Middle French (French: moyen français) is a historical division of the French language that covers the period from (roughly) 1340 to 1611. It is a period of transition during which:
the French language becomes clearly distinguished from the other competing Oïl languages, which are sometimes subsumed within the concept of Old French (ancien français)
the French language is imposed as the official language of the kingdom of France in place of Latin and other Oïl and Occitan languages
the literary development of French prepares the vocabulary and grammar for the Classical French (français classique) spoken in the 17th and 18th centuries.
History
The most important change found in Middle French is the complete disappearance of the noun declension system (already underway for centuries). There is no longer a distinction between nominative and oblique forms of nouns, and plurals are indicated simply with an s. This transformation necessitates an increased reliance on the order of words in the sentence, which becomes more or less the syntax of modern French (although there is a continued reliance on the verb in the second position of a sentence, or "verb-second structure", until the 16th century).