Old City of Toronto
This is subjective term as there always seems to be an "Old City of Toronto" coming up in conversation when talking about things within this "Toronto", and as always, context is key to understanding which of the numerous "Old City of Toronto"'s one is referring to.
Pre-Amalgamation
Nowadays, most often, "Old City of Toronto" is understood as being the pre-amalgamation City of Toronto that existed until 1997 within the Regional Municipality of Metropolitan Toronto.
From 1998 and into the first few years of Amalgamation, the New City of Toronto was referred to as The Megacity.