According to NICE, 3 or more loose or liquid stools in a day (or more frequently than is normal for the individual) counts as diarrhoea.
Persisting for more than 14 days makes it chronic.
Acute typically gastroenteritis. Presence of blood and/or mucus suggests more invasive inflammation, viz colitis.
In kids, can occur with pretty much any illness!
Vomiting with diarrhoea makes a primary gut cause more likely, but still not specific.