What prefix signifies "none" or "not at all"?

The prefix "nulli-" indicates the absence or negation of something, which translates to "none" or "not at all." This prefix is commonly used in medical terminology to denote the absence of certain conditions or characteristics, such as "nullipara," which refers to a woman who has never given birth.

In contrast, the other prefixes have different meanings that do not reflect the concept of absence. For instance, "mono-" refers to one or single, "pan-" signifies all or everywhere, and "semi-" means half or partially. Therefore, "nulli-" is the appropriate choice when referring to the idea of nothing or the complete lack of something in a medical context.