Hyperglycemia or Hyperglycæmia, or high blood sugar, is a condition in which an excessive amount of glucose

Glucose

Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 circulates in the blood plasma

Blood plasma

Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...

. This is generally a glucose level higher than 13.5mmol/l (243mg/dl), but symptoms may not start to become noticeable until even higher values such as 15-20 mmol/l (270-360 mg/dl). However, chronic levels exceeding 7 mmol/l (125 mg/dl) can produce organ damage.

The origin of the term is Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

: hyper-, meaning excessive; -glyc-, meaning sweet; and -emia, meaning "of the blood".
Glucose levels are measured in either:

1. Milligrams per decilitre (mg/dl), in the United States and other countries (e.g., Japan, France, Egypt, Colombia); or
2. Millimoles per litre (mmol/l), which can be acquired by dividing (mg/dl) by factor of 18.

Scientific journals are moving towards using mmol/l; some journals now use mmol/l as
the primary unit but quote mg/dl in parentheses.

Glucose levels vary before and after meals, and at various times of day; the definition of "normal" varies among medical professionals.