Hypo is short for hypoglycaemia.
This means the blood sugar has dropped so low that efficient brain function is at risk. The blood sugar of normal non diabetics is usually not under 3.3. For a bit of a buffer, diabetics are advised to correct blood sugars lower than 4. Normal blood sugar in fasted fit young people or between meals is usually 4.6. This can rise after meals but not usually more than 10 no matter how much carbohydrate they may eat. Diabetics try to limit post meal blood sugars to less than 8 for strict control and less than 10 for more relaxed control.
In the USA, the usual term for hypos is simply "low".
