Which word describes the act of changing or causing to change in character or composition significantly?

The word that best describes the act of changing or causing to change in character or composition significantly is "transform." This term implies a thorough or dramatic change, suggesting a fundamental change in form, character, or appearance.

"Alter" also indicates a change but typically suggests a smaller adjustment rather than a significant transformation. "Modify" conveys making minor changes or improvements, often without altering the essential nature of something. Meanwhile, "adapt" refers to adjusting or becoming suited to new conditions but does not necessarily imply a significant change in a fundamental way.

In understanding the nuances of these terms, "transform" specifically highlights the idea of a major change, aligning perfectly with the question's emphasis on significant alteration.