In chemistry, to be dissolved in water. Denoted by putting the subscript (aq) after the chemical symbol. For instance, salt disassociating in water to form sodium and chlorine ions:
NaCl(s) --> Na+(aq) + Cl-(aq)
All acids and alkali are defined as being aqueous. So we don't say "aqueous hydrogen chloride". Instead we say "hydrochloric acid".