After the unsettling certification of a city proper, I've been on a search to see what other categories shine a streetlight on built-up areas. Metropolitan areas are difficult to measure but are roughly designated as including the labor market area and the surrounding commute, with the caveat that the surrounding region must be of minimal agricultural focus, and include a large number of commuters into the main urban area.
Despite this sub-sophistry, the most populated metropolitan areas are much the same as the most populated cities, only in a slightly different order. Tokyo is the largest and the two not figured in 'cities with the highest population' are Mumbai (20,748,395) and Osaka-Kobe-Kyoto (19,342,000)
Here's a word you don't hear every day: agglomeration. An agglomeration is a large group or pile of different things, but in an urban planning sense, an urban agglomeration is 'the population contained within the contiguous territory without regard to administrative boundaries or commuter flows' according to Wikipedia. So it does not even have the restriction in definition of a 'metropolitan area'
I was going to condense all the population aggregation into one thread but I've encountered a setback: I've seen my Wikipedia source virtually change before my eyes so what you see is a 'before' snapshot. Okay, maybe I won't attempt conurbation but before I go on with the other significant measures, I need to take stock of what changes have been made around me.