What is the definition of classical singing? When people refer to themselves as singing using classical technique what do they mean?
Classical singing is usually referring to the idea of choral works performed in a classical style. We wouldn't consider Bruno Mars to be a classical singer, for example. Not even Elvis. Opera, I believe, is another area of classical singing. Classical singing generally has a strong emphasis on homogenous technique and tone, where modern/contemporary styles tend to eschew those values, instead going for whatever works. Classical singing also tends to be accompanied by classical music, unsurprisingly. If you grab a group of ten classical singers of the same voice type and ask them to sing the same thing, it should be hard to tell them apart. Ten pop singers w(sh)ould not sound so identical.
My best guess is, that completion of a classical singers training, likely even involving a degree is intended here. Classical is due to the fact, that in the sector of classical music this is a strong requirement and not replaceable by any amount of other practice.