I define it as identifying, promoting, and preserving cultural differences among classes of people within a single nation.
I think it is virtually certain to lead to factionalism and ultimately the break up of the nation. Which may be okay. But I think multiculturalism leads almost inevitably to the break up of any nation that practices it so if that is NOT what you want, then you need to trun around and start marching the other way.
I think the culture of a nation needs to adapt, adopt, grow, and change with different cultural influences, including from immigrants, but it should always be trending toward homogeneity. Otherwise it will be trending toward civil war.