# What kind of definitions need be introduced in a PhD thesis? [closed]

For all the terminology in mathematics, I am not sure where to draw a line between those which need to be introduced in a PhD thesis and those which do not. For example, I suppose that in a PhD thesis, no one will bother to give the definition of group, or normal subgroup, etc. but probably commutators as it can be defined in two different ways, namely $x^{-1}y^{-1}xy$ and $xyx^{-1}y^{-1}$. For braid group one probably will write down the definition anyway, even though it is well known among topologists. I am not sure about some terminologies such as the lower central series: does it need to be introduced?

Where do we draw the line?