Fundamentalism is the persistent refusal to apply the three central concepts of ethics. It is extreme fanaticism, which turns against ethics.
Fundamentalism restricts itself to a single source of knowledge. Fundamentalism refuses to examine this single source of knowledge from the prespective of changed circumstances and new insights.
Fundamentalists are not open to changing their mind about ethical decisions. As only a single, static source of knowledge is recognised as the universal solution, as the universal salvation, all other knowledge is excluded.
Fundamentalism refuses to verify knowledge. It is a refusal to think.
These three central concepts of ethics precisely describe the original meaning of religion:
Latin relegere: "to consider carefully, to contemplate, to think about repeatedly, to be thoughtful in view of an important issue.
Centuries later, the derivation of the word religion was held to be:
Latin religare: "to tie to something, to bind fast", which tends in the direction of the definition of fundamentalism.