Seattle has many distinct neighborhoods like Capitol Hill, one of Seattle's oldest and most historic neighborhoods, or Queen Anne, or even First Hill which used to be called Profanity Hill.

Reference to Profanity Hill in "The Seattle Times" from July 15th, 1896

However, doing a quick Wikipedia search about the neighborhoods of Seattle would tell you that some of the neighborhoods aren't actually that distinct.

"Unlike some cities, Seattle doesn't set neighborhood boundaries. They're ever-changing," said Brent Crook, a division director in the city's Department of Neighborhoods. ... "I've seen my area go from the 'CD' to 'Madrona' to 'Greater Madison Valley' and now 'Madrona Park,' " said Natasha Jones, Department of Neighborhoods spokeswoman.

Recently I had a question, why is South Lake Union called "South Lake Union" and not called "Southlake"? The neighborhoods of Eastlake and Westlake exist and Google Maps claims there is a neighborhood called Northlake which is also a part of Wallingford.

The "neighborhood" of Northlake
The "neighborhood" of Wallingford which encompasses all of Northlake

Wallingford, like most neighborhoods in Seattle, also does not have distinct boundaries. According to one source

One survey surprise revealed that most of the families interviewed living in the four-block swath between Stone Way and Aurora Avenue did not consider themselves Wallingfordians, but more often citizens of Fremont. Thereafter the social scientists agreed that households west of Stone Way would be “dropped from future progress plans affecting Wallingford.”
Actually there is a unique name for this uncertain district just west of Stone Way. Some call it Freford, parallel to Frelard, the name given the uncertain area sharing Fremont and Ballard between 3rd and 8th avenues NW. Border anxieties may be expected of Fremont, the neighborhood that describes itself as “The Center of the Universe.” To complicate matters further, some residents of "Freford" refer to it as "Wallmont."

Anyway, we have the "lake" neighborhoods but South Lake Union is different, it's not called "Southlake". Why? To help me find an answer to this question I asked the Seattle Public Library. Less than a day later I had an answer.

The neighborhoods of Eastlake and Westlake contain their respectively named streets, Eastlake Avenue and Westlake Avenue. These streets are ancient, existing on maps going back to 1905.

Baist's Real Estate Atlas of Surverys of Seattle, Wash - Plate 7. Created 1905

These streets ran on the east and west side of Lake Union. So was there a Southlake Avenue? Actually there was for a short period of time.

Baist Atlas. Created 1912
The first reference to Southlake Avenue in "The Seattle Daily Times" from January 7th, 1911

From somewhere between 1911 and 1924 Southlake Avenue was a real street running just south of Lake Union.

The last reference to Southlake Avenue in "The Seattle Daily Times" from August 8th, 1924

However, by 1924 Southlake Avenue had been renamed to Fairview Avenue North.

Kroll's Atlas of Seattle Map 7W ; west 1/2 29-25-4. Created 1920

Directly quoting Jade from the Seattle Public Library

In short, it looks like South Lake Union was the more frequently used reference for the area and was in use before Southlake Ave. existed. Since Southlake Ave. was quickly incorporated into Fairview Ave., it likely didn't stick around long enough to become a common reference point for the neighborhood in the way that Westlake and Eastlake did.

So this is all well and good but why "South Lake Union" instead? Well, South Lake Union is south of Lake Union. It's really that simple. The first reference to South Lake Union in The Seattle Times is a real estate ad from 1906.

South Lake Union real estate. "The Seattle Daily Times" from March 11th, 1906

There's also a map of the area from a 1922 article.

South Lake Union District. "The Seattle Daily Times" from April 30th, 1922

However, the article below it refers to the area as "the district at the south end of Lake Union"

South Lake Union District. "The Seattle Daily Times" from April 30th 1922

References to South Lake Union drop dramatically between 1930 and 1959. There are only 2 references between 1930 and 1939, 3 references between 1940 and 1949, and 1 reference between 1950 and 1950. This is compared to 93 references between 1920 and 1929. They only start to rise again starting in the 1960s.

References to South Lake Union in The Seattle Times

So what area is South Lake Union? I would consider its south boundary to be Denny Way, its north boundary to be Lake Union, its west boundary to be Aurora Ave N and its east boundary to be I-5. Here's what Google thinks

South Lake Union defined by Google

Wait hold on, Cascade? There's another neighborhood inside South Lake Union? Well according to Bing this is Cascade

Cascade as defined by Bing

It seems like Bing is going off the city of Seattle's data. At some point the city defined Cascade as a neighborhood encompassing the neighborhoods of Westlake, South Lake Union, and Eastlake.

Cascade as defined by the city of Seattle

Some textual references "match" the Google maps definition of Cascade.

Seattle's Cascade neighborhood, resting at southern end of Lake Union and bounded by Fairview Avenue N, Eastlake Avenue E, and E Denny Way
The Cascade Neighborhood, as defined for this study, includes the area bounded by the eastern side of Fairview Avenue to Interstate 5 and from the Roy Street to Denny Street.
For the purposes of this paper, I will define the Cascade district as follows: the district lies between Denny Way and Lake Union in the north-south direction and between Fairview Avenue North and Eastlake Avenue East in the east-west direction.

There is also another a link on the city website that gives some information about where Cascade might be. A listing of Seattle Historical Sites in the Cascade neighborhood lists locations between Fairview Ave N on the west, Eastlake Ave E on the east, and Denny Way on the south. However the same historical site listing also has a reference to the South Lake Union neighborhood which lists locations between Dexter Ave N on the west, Fairview Ave N on the east (with one building on Eastlake Ave E), and Denny Way on the south. Notice that these two area definitions don't overlap, the boundary is Fairview Ave N.

So where did the name "Cascade" come from. Well there's the Cascade Range which is probably where the name for the Cascade School came from. The Cascade School opened in 1894 just south of Lake Union.

The first reference to the Cascade School in "The Seattle Times" from April 24th, 1896.

There's also a reference to the area near the school in 1906.

"in vicinity and west of Cascade School" in "The Seattle Sunday Times" from September 16th 1906

It's hard to know how many references there are to the neighborhood of Cascade in The Seattle Times because of the mountain range area but there's one reference to the "Cascade district" in 1909 and one reference in 1938. Any references to the "Cascade neighborhood" start in the 70s and those are few and far between compared to South Lake Union.

References to Cascade neighborhood or Cascade district in The Seattle Times

The Cascade School was severely damaged in the 7.1 magnitude 1949 Olympia earthquake on April 13th, 1949 and it was torn down in 1955. The school playground is became a Seattle park, Cascade Playground.

There are two articles from HistoryLink, "Seattle Neighborhoods: Cascade and South Lake Union — Thumbnail History" from 2001 and "Lake Union (Seattle) Tour" from 2007, that go into the history of the area and there's a paper called "2003 Cascade Historic Survey Buildings, Objects & Artifacts" commissioned by the city that also goes into the history of the area but they didn't really give much info on how or why Cascade and South Lake Union are two different neighborhoods.

So, what is South Lake Union? South Lake Union is a neighborhood in Seattle just south of Lake Union. It has Westlake to its northwest and Eastlake to its northeast. South Lake Union isn't called "Southlake" because the street Southlake Avenue only existed for approximately 13 years between 1911 and 1924 compared to Westlake Avenue and Eastlake Avenue which have existed since at least 1905. South Lake Union is either contained by, adjacent to, or contains the neighborhood of Cascade, depending on who you ask.