Euglycemia, also known as normoglycemia, simply refers to "normal" or "nice" blood glucose levels, that is, not too high (hyperglycemia) or low (hypoglycemia). Regulation attempts to achieve euglycemia for a substantial portion of the day, in a predictable way. Tight regulation attempts to remain in euglycemia at all times.
The upper and lower boundaries of the euglycemic zones are fuzzy, so no exact definition would always be correct. Diabetic definitions of euglycemia are a bit different from non-diabetic ranges, so we won't offer a single definition here. For possibilities, see: