Potassium: The major positive ion (cation) found inside cells. The chemical notation for potassium is K+. The proper level of potassium is essential for normal cell function. An abnormal increase in potassium (hyperkalemia) or decrease in potassium (hypokalemia) can profoundly affect the nervous system and heart, and when extreme, can be fatal. The normal blood potassium level is 3.5'5.0 milliEquivalents/liter (mEq/L), or 3.5 international units.