A “cocktail party” is simply a “party” where “cocktails” ( as well as other refreshments) are served. The Missouri newspapers in March 1917 credited a Mrs. Walsh of St. Louis with originating something new in the “cocktail party.”
 
The 1917 publicity certainly popularized the American institution of the cocktail party, but prior citations exist. As early as 1838 and again in 1873 and 1893, the “cocktail party” was sort of a new political party. The term “cocktail party” in its proper use had been cited in 1903 (Illinois) and 1915 (California).
 
   
Wikipedia: Cocktail party
Cocktail party is a party where cocktails are served. Women may choose to wear what has become known as a cocktail dress.
 
Although many believe the inventor of the cocktail party to be Alec Waugh of London, who in 1924 found a need for this pleasant interlude before a dinner party, an article in the St. Paul Pioneer Press that ran in May 1917 credits Mrs. Julius S. Walsh Jr. of St. Louis, Missouri. Mrs. Walsh invited 50 guests to her house on a Sunday at high noon for a one hour affair. “The party scored an instant hit,” the St. Paul Pioneer Press declared. And within a matter of weeks, cocktail parties became “a St. Louis institution”.
 
Alec Waugh noted that the first cocktail party in England was hosted by war artist Christopher Nevinson in 1924.
 
(Oxford English Dictionary)
cocktail party n. a party, esp. one intended for social conversation, at which cocktails are served, usu. together with other alcoholic drinks, savoury snacks, canapés, etc.
1928 D. H. LAWRENCE Lady Chatterley’s Lover vi. 67 She almost wished she had..made her life one long *cocktail party and jazz evening.
1950 T. S. ELIOT (title) The cocktail party.
1950 T. S. ELIOT Cocktail party I. i. 12 The only reason for a cocktail party For a gluttonous old woman like me Is a really nice tit-bit.
 
25 January 1838, Baltimore (MD) Sun, pg. 2:
The gin sling and cocktail party had such power in the States that they controlled the government.
 
5 August 1873, Cincinnati (OH) Daily Gazette, pg. 7:
Despite all prophecies, the cocktail party will receive no support at all from the dissatisfied regular Republicans. The latter would rather vote with the Democratic party.—Volksfreund.
 
26 September 1893, Baltimore (MD) Sun, “From Washington,” pg. 1:
This attitude of Senator Butler toward the administration and the measures it favors may endear him to the Palmetto “cocktail” party that is now in the ascendency in his State, but the genuine democrats of South Carolina,...
   
16 November 1903, Decatur (IL) Daily Review, pg. 1, col. 3:
KILLING RESULTS FROM AN
AFTER-THEATER COCKTAIL PARTY
 
5 February 1915, Oakland (CA) Tribune, pg. 7, col. 1:
He presented these as proof of the charges made last week regarding a supposed “cocktail party” held in the firehouse kitchen, charges that certain telephone calls had been charged to the city by firemen communicating with a certain apartment house in San Francisco, and other charges that the fire department did not do its duty at a recent Piedmont fire.
   
30 March 1917, Kansas City (MO) Star, section 2, pg. 10B:
GAVE A “COCKTAIL PARTY”
AND MRS. WALSH OF ST. LOUIS HAS
ORIGINATED SOMETHING NEW.
The Home Before Sunday Dinner Was
Devoted to the Drinking of Ap-
petizers at the Private Bar
of a Boulevard Home.
Positively the newest stunt in society is the giving of “cocktail parties.”
 
The cocktail party is a Sunday matinee affair which originated in St. Louis.
 
Mrs. Julius S. Walsh, jr., a leader in social activities there, is responsible for the innovation.
 
Mrs. Walsh introduced it last Sunday, with the first cocktail party in society’s history. Invitations were issued to fifty. The guests were divided into two classes—those who went to church in the forenoon and those who devoted their time to motor promenade of the boulevards. Then at high noon they gathered at the Walsh home on Lindell Boulevard for the hour’s “interregnum preceding 1 o’clock dinner.”
 
AN INSTANT HIT.
The party scored an instant hit. Mrs. Walsh’s home is equipped with a private bar. Around this the guests gathered and gave their orders in a white coated professional drink mixer who presided behind the polished mahogany. If a woman guest who had been driving all forenoon in her limousine, and was a little chilled in consequence, felt the need of a drink with an extra kick in it, she ordered a Sazarac cocktail. Others, of course, preferred a Bronx or a Clover Leaf, and a few who had been to church were old fashioned enough to order a martini or a Manhattan.
 
And as long as the professional drink mixer was there to fill all orders, other beverages than cocktails were in demand. Highballs, some with Scotch and some with rye or Bourbon whisky, gin fizzes—ordered because the spring morning hinted of coming summer—and at least one mint julep for a former gentleman of Virginia, were handed out over the private bar.
 
COMES TO STAY.
That the cocktail party already is a St. Louis institution, filling a long felt Sunday want in society circles there, and that the party at which Mrs. Walsh was hostess was so merry and so jolly as to approach in hilarity the famous early morning eggnogg parties popular in the same city a decade ago, is vouched for by the St. Louis newspapers.
 
In the meantime Mrs. Walsh, because of her innovation, has become more of a social celebtrity in St. Louis than ever.
 
Google Books
Over Here:
Impressions of America By a British Officer
By Hector MacQuarrie
Philadelphia, PA: J. B. Lippincott Company
1918
Pg. 42:
About cocktail parties—well, I don’t quite know. I rather suspect that they are bad things. They always seem to remind me of the remark in the Bible about the disciples when they spake with tongues and some one said: “These me are wine bibbers.” I rather think that cocktail parties are a form of wine bibbing. Still they play an important part in the life of some people, and I had better tell you about them. As a matter of fact, quite a large number of people at a cocktail party don’t drink cocktails at all, and in any case, they are taken in a very small shallow glass. The sort one usually gets at a cocktail party is the Bronx or Martini variety. The former consists, I believe, largely of gin and orange juice and has a very cheering effect. People mostly walk about and chat about nothing in particular. They are generally on their way home from church and nicely dressed.
sort one usually gets at a cocktail party is the Bronx or Martini variety. 
   
7 April 1921, Massillon (OH) Evening Indepedent

The first broadside was fired Wednesday by Wells in his testimony to the effect that his marriage to Mrs. Wells came about as a result of a cocktail party about eight weeks after he had met her.
 
Time magazine
New Plays
Monday, Mar. 08, 1926
(...)
Mama Loves Papa. A wobbly little comedy about marriage appeared under this awkward title and was not very fervently applauded. The two young things of the title phrase stumbled innocently and separately into a wicked cocktail party in the city.
   
13 April 1926, Los Angeles (CA) Times, pg. 1:
BUTLER TELLS COURT OF FAMOUS COCKTAIL PARTY
General Traces Gay Evening Until He Says Colonel Was Assisted Out of Coronado Drunk
By Floyd A. Healey