Here's why:

* Brønsted-Lowry definition: HCl is an acid because it donates a proton (H+) when dissolved in water. The reaction looks like this:

```

HCl(aq) + H2O(l) ⇌ H3O+(aq) + Cl-(aq)

```

* Arrhenius definition: HCl is an acid because it produces hydrogen ions (H+) when dissolved in water.

Let me know if you'd like to learn more about acids and bases!