Sodium: The major positive ion (cation) in the fluid surrounding cells in the body. The chemical notation for sodium is Na+. When sodium is combined with chloride, the resulting substance is a crystal called table salt. Excess dietary sodium is largely excreted in the urine, but too much salt in the diet tends to increase the blood pressure. Too much or too little sodium in the blood (called hypernatremia or hyponatremia respectively) can cause cells to malfunction, and extremes can be fatal. Normal blood sodium level is 135'145 milliEquivalents/liter (mEq/L) or 135'145 millimoles/liter (mmol/L) in international units.