In Warhammer, what is it called when you create new models from bits of other ones?

In particular, what's it called when you create a Khaos demon out of bits of other models or stick green stuff all over something to make it look possessed? Is it conversion? I can't remember. Or is it only called conversion when you're creating a particular unit from pieces of another one?In Warhammer, what is it called when you create new models from bits of other ones?
Most people call it converting; it's when you remodel a miniature.In Warhammer, what is it called when you create new models from bits of other ones?
its called a re birthed claydoll demon