According to Bronsted-Lowery Concept, An Acid is a specie which can donate H⁺ and Base is a specie which tends to accept H⁺.
In equation given as,
HCl₍g₎ + H₂O₍l₎ → H₃O⁺₍aq₎ + Cl⁻₍aq₎
In this reaction HCl is loosing proton and converting into conjugate base Cl⁻ and H₂O is accepting proton and converting into conjugate base H₃O⁺.
Result:
Acid = HCl
Base = H₂O