Hyperglycemia refers to an elevated level of glucose in the blood, commonly associated with Diabetes mellitus. It occurs when the body either doesn't produce enough insulin or cannot effectively use the insulin it produces, leading to increased blood sugar levels. Symptoms can include increased thirst, frequent urination, fatigue, and blurred vision. If left untreated, severe hyperglycemia can lead to serious complications, such as diabetic ketoacidosis or hyperglycemic hyperosmolar state.