Native Lushootseed speakers have several names for Mt. Rainier, including təqʷubəʔ, which has been anglicized as 'Tahoma', 'Tacoma', and 'Tacobet', and this is where the city of Tacoma gets its name from. The name means means "snow-covered mountain" and is an apt description for this beautiful stratovolcano which has a snow-capped peak year-round. But why do locals call it "The Mountain" when there are so many mountains around, and besides, it's a volcano and not just a mountain? While it's true that Mt. Rainier is technically a part of the Cascade Mountain range, it seems to stand apart from the rest of the range, looming close over the landscape. Because it seems to stand alone, it's become "The Mountain" in everyone's minds, as opposed to "the mountains" (emphasis on the plural) which is usually used to refer to the Cascades to the east, and less often to the Olympic range on the peninsula to the west across the waters of the Puget Sound.
Tahoma, or Mt. Rainier, is a symbol of the region and a beautiful sight, so much so that no matter how long you've lived in the area, seeing it come out is still a marvel worth commenting on, hence why you hear people say that the Mountain is out.