There's two forms of the word tank that are applicable here.
The first is the role Tank. This role is characterized as using defense and utility to control the battlefield.
The second is the form that refers to a character with defensive stats. They simply have to have high defenses to fit this definition. Some characters are naturally better at it than others, but when built correctly any champion can fit this form of the word.
Neither form of the word touches on damage dealt. You're looking at Tank from the perspective of damage, and that point of view is inherently flawed. The role Tank is often balanced around low damage/high utility, but it is not a requirement, and Nasus does not fit that role in any case.
Damage dealers and tanks are supposed to overlap, because the traits that make them what they are are not mutually exclusive. If you really want to talk about how things overlap there are other aspects of a character you have to add in, like damage type and mobility, otherwise you're not looking at the whole picture. If it wasn't 6am for me I might be able to explain this a little better, but hopefully you understand.
