This basically corresponds with topography. The hill gets especially steep after a few blocks and is generally just cliff-face in certain areas. That line, which is sometimes not cliff-face especially toward the East End, nonetheless forms the mental barrier between downtown near sea level and the rest. Uphill, where it's mostly residential, is center city. Downhill, where it's mostly commercial, is "The Downtown". The view up Beck's Cove toward George Street demonstrates this well because the divide between the two levels of the core is obvious:
April 14, 2017 by
R C, on Flickr
Livingstone Street is another good example. It's the one running across the photo. Foreground isn't downtown, background on the other side of that street is.
Christmas Parade by
R C, on Flickr
Just "downtown" can be somewhat broader than that, typically corresponding with the crest of the first hill up from the harbour. In that instance, the absolute edge of "downtown" would run along Pennywell, Merrymeeting, Bonaventure, and Military. It's much bigger. For example, in that photo above, now the foreground as well would be "downtown". But that tends to be a view only held by people not from the core. The divide is strong enough that one of the oldest jokes here is a tourist stopping someone on Kenmount Road and asking how to get to Signal Hill. Local thinks and minute and says, "You can't get there from here" lol.
The Basilica of St. John the Baptist near the centre of this photo is at the crest of the hill. To the left side of the photo (which is downhill and heads toward the harbour) could be considered "downtown". To the right (Georgestown neighbourhood, on to Churchill Park neighbourhood) never would. And just to hammer in how not-wide downtown is, cut off from the left side of this photo is only about 3-4 blocks of city before the harbour, that's as wide as downtown gets):
Bird's Eye View # XXXI ... ; (c)rebfoto by
rebfoto, on Flickr
For urbanists like us, it's really only two streets and the countless little ones that connect them - Water and Duckworth. They're not broad - it's only say three blocks wide - but they're very long, stretching more than the length of the harbour itself and urban for the majority of that distance.
December 13, 2015 by
R C, on Flickr