What is the term for substances that are not effective for allowing X-rays to pass through?

The correct term for substances that do not allow X-rays to pass through is "radiopaque." Radiopaque materials absorb or block the passage of X-rays, resulting in a visible image on X-ray films. This characteristic is crucial in medical imaging as it helps differentiate various structures within the body. For instance, bones are considered radiopaque because they absorb more X-rays compared to surrounding tissues, making them appear white on X-ray images.

In contrast, radiolucent substances are those that allow X-rays to pass through with little absorption, leading to a darker appearance on the film. Opaque is a broader term that indicates something that cannot be seen through, which may not necessarily apply specifically to X-ray imaging. Transparent materials, on the other hand, allow light (and to some extent, X-rays) to pass through clearly, which is opposite to what is described in the question.

Thus, radiopaque is the precise terminology for materials that do not allow X-rays to pass through effectively, making it the correct answer.