a prefix used to mean “under” or “less than.” I remember it as “hypo -> below” (as opposed to its opposite, “hyper-” (“hyper-over”). We can stick hypo- in front of a lot of different terms (e.g. hypotonic, hypoosmotic). You might have heard it in more “normal talk” in the context of “hypothermia” which is where someone’s body temperature is below what it should be, or a “hypodermic needle” which injects you under the dermis of your skin.