Formula fed baby poop watery is it diarrhoea?

What is considered as diarrhea for baby? Full formula. If baby only poop once on that day and its dark yellow watery, is it already considered diarrhoea? If baby poops watery stool 3 times on that day ? All mostly after milk and large amount of poop. But most of the days the poop is normal only some days its like that.