Lewis Base: A Lewis base is a species that can donate an electron pair. In the Lewis model, NH$_3$ is considered a base because it has a lone pair of electrons on the nitrogen atom that can be donated. This lone pair can interact with a Lewis acid (a species that can accept an electron pair) to form a coordinate covalent bond.
Bronsted-Lowry Base: A Bronsted-Lowry base is a species that can accept a proton (H$^+$). In the Bronsted-Lowry model, NH$_3$ is not considered a base because it does not have any acidic protons that can be donated.
Arrhenius Base: An Arrhenius base is a species that produces hydroxide ions (OH$^-$) when dissolved in water. In the Arrhenius model, NH$_3$ is not considered a base because it does not produce OH$^-$ ions when dissolved in water. Instead, it reacts with water to form ammonium hydroxide (NH$_4$OH), which is a weak base.
Therefore, NH$_3$ is only considered a base in the Lewis model, but not in the Bronsted-Lowry or Arrhenius models.
