What is the medical terminology for decreased or lack of?

The medical terminology for decreased or lack of is "hypo-." This prefix is used to denote a deficiency or lower than normal level of a substance or function in the body, such as in "hypoglycemia" (low blood sugar) or "hypotension" (low blood pressure). Conversely, "an-" or "a-" can also indicate a complete lack of something, as in "anemia" (lack of healthy red blood cells) or "aseptic" (absence of infection).