Why is guanidine considered a strong base? I've studied $\mathrm{p}K_\mathrm{a}(\ce{H2O}) = 15,$ and anything with a $\mathrm{p}K_\mathrm{a}$ lower than should be an acid. Guanidine has a $\mathrm{p}K_\mathrm{a}$ of $13.8$ and is being called by Wikipedia a strong base. Why is it so?
According to Wikipedia, ammonia is a weak base. Then why is its $\mathrm{p}K_\mathrm{a}$ much higher than the $\mathrm{p}K_\mathrm{a}$ of guanidine?
Wikipedia notes that if a base (in water solution) has a conjugate acid whose $\mathrm{p}K_\mathrm{a}$ exceeds $13,$ it's a strong base.  The criterion is (apparently) based on complete or at least predominant dissociation up to $0.1$ molar.  Guanidine makes it, ergo a strong base.