The chloride anion,
Dissolving hydrogen chloride in water will get you hydrochloric acid, which is essentially an aqueous solution of hydrogen chloride.
#"HCl"_ ((g)) -> "HCl"_ ((aq))#
Hydrochloric acid is a strong acid, which means that it ionizes completely in aqueous solution to produce two ions, the hydronium cation,
#"HCl"_ ((aq)) + "H"_ 2"O"_ ((l)) -> "H"_ 3"O"_ ((aq))^(+) + "Cl"_ ((aq))^(-)#
Hydrochloric acid solutions have very high concentrations of hydronium cations. Consequently, hydrochloric acid solutions will have a very low
When it comes to hydrochloric acid solutions, we're usually more interested in the hydronium cations, so I'm guessing that the "other ion" you mentioned would be the chloride anion.