Under; below normal; slightly.
[Greek hupo, under.]
A hypodermic syringe (Greek derma, skin) is one that injects a drug below the skin; hypotension is abnormally low blood pressure; hypoglycaemia is deficiency of glucose in the bloodstream; hypothermia is the condition of having a low body temperature, usually dangerously so.
Several words were imported from Greek, and the sense in these is usually figurative: hypothesis, a provisional explanation, comes from Greek hupothesis, foundation; hypocrite, someone claiming to have better standards or beliefs than is the case, is from Greek hupokrinesthai, to play a part or pretend, from krinein, to judge; hypochondriac, somebody abnormally anxious about their health, derives from Greek hupokhondria, the soft body area below the ribs in which melancholy was once thought to be based.
In chemistry, the prefix refers to a compound containing an element with an unusually low valency, as in the bleaching agent hypochlorite, or the photographer's hypo from sodium hyposulphite (now more usually sodium thiosulfate).
The prefix is sometimes confused with hyper-.