Why is ammonia considered a base?
It's a Bronsted base because it can accept a proton.
The Brondsted-Lowry definition defines bases as substances that accept protons, or
We can see that
The forward reaction doesn't occur that readily, so, although ammonia is a base, it's a weak base.
If we were to be really specific, ammonia sometimes also behaves as an acid by donating an
This would make ammonia amphoteric, which means that it can behave as both an acid and a base.
BUT, the above reaction (when ammonia is behaving as an acid) happens at a way lower rate than when ammonia accepts a proton, so we call it a base.