Definition:
Something already exists. A novel, a play, a true story, a comic book. Someone decides it belongs on screen. The process of taking that source material and reimagining it in cinematic form is an adaptation. The source provides the raw material. The film has to figure out what to do with it. Those are two very different jobs.
What It Means in Movies:
Adapting a book is not translating it. It is rebuilding it using completely different tools. Written narrative lives inside a character's head. Film has to find a way to put that interior life on screen without just narrating it. Plots get condensed. Characters get merged or cut entirely. The creative team's job is not fidelity. It is finding what the story actually needs to function in a new medium.
Why It Matters:
Hollywood runs on adaptations. The reason is not mysterious: they come with a built-in audience. The marketing risk is lower when people already know the title. But that familiarity cuts both ways. Some of the most interesting creative arguments in contemporary cinema happen in the space between what a book was and what its adaptation chose to become.
Example:
Ang Lee's 2012 film Life of Pi adapts Yann Martel's novel of the same name: a book whose central narrative unfolds almost entirely inside its protagonist's mind. The film had to find visual and cinematic solutions to a story that, on the page, actively resists them. Whether it succeeded is still argued about. That is probably the most honest thing you can say about any adaptation.