What is the definition of a mass shooting?

In that case, wherever you are getting that number I'm sure they provide their own definition or source, or you can contact whoever published it to give you their source.

However, I'm sure going into a church and shooting everyone in it simply because they were "black", or opening fire on any large gathering simply because they are there, deserves it's own category.
So yes the above is classified as a mass shooting
https://en.m.wikipedia.org/wiki/List_of_mass_shootings_in_the_United_States_in_2019