According to the arrhenius theory an acid is a substance that

An acid is a substance that releases H⁺ in aqueous solution.

Explanation:

There are different acid-base theories, such as Arrhenius, Bronsted-Lowry, Lewis, etc.

According to the Arrhenius theory, an acid is a substance that releases H⁺ in aqueous solution.

HCl(aq) → H⁺(aq) + Cl⁻(aq)

On the other hand, according to the Arrhenius theory, a base is a substance that releases OH⁻ in aqueous solution.

NaOH(aq) → Na⁺(aq) + OH⁻(aq)

According to the theory, an Arrhenius acid is a substance that gives hydrogen ion, (or protons) as the only positively charged ions, when dissociated in water.

Explanation:

What this means is that acids increase the concentration of protons in water to form hydronium ions,as opposed to the idea that there aren't free hydrogen cations floating around in aqueous solution.

The Arrhenius definitions of acidity and alkalinity are restricted to aqueous solutions and refer to the concentration of dissolved ions.

if you're asking what comes out, it's steam.