* Structure: HONH₃⁺ is the conjugate acid of hydroxylamine (HONH₂). It has an extra hydrogen ion (H⁺) attached to the nitrogen atom.

* Brønsted-Lowry Definition: According to the Brønsted-Lowry definition, an acid is a substance that donates a proton (H⁺), while a base accepts a proton.

* Proton Donation: HONH₃⁺ can donate its extra proton, becoming HONH₂ again.

In summary: HONH₃⁺ is an acid because it can donate a proton.