Fanfiction is defined as a fan of a particular series drawing inspiration from the characters, original storyline, and environment from a series they enjoy. From this, the fan creates their own original story and timeline for their characters to follow. Additionally, Fanfiction is typically for the fan’s enjoyment.
From my understanding, adaptation is transferring a story to a new medium which typically takes the form as film. Understandably, narratives that have been adapted undergo numerous changes which alter the original environment that was created. In some cases, these changes can be against the “agreed” narratives that fans enjoy and were changed for financial reasons when producing a film. \
The key difference between Fanfiction and adaptation is that Fanfiction is produced for the enjoyment of others who enjoy a particular series, while an adaptation oftentimes takes the form a film made for enjoyment but also financial gain.