ChlorideCl
The salt partner to sodium, used mainly to check the blood's acid balance.
Chloride is the negatively charged ion that accompanies sodium, and the two usually move together. It is measured on almost every basic panel because it helps balance the electrical charge of the blood.
It matters mostly in combination. Comparing chloride and bicarbonate against sodium produces the anion gap, a calculation used to work out whether the blood is too acidic and, if so, roughly why.
In practice a chloride result is rarely interpreted on its own. It shifts with hydration, with prolonged vomiting, and with the same conditions that move sodium.