Entry in progress—B.P.
 
Wikipedia: Lager
Lager (German: storage) is a type of beer made from malted barley that is brewed and stored at low temperatures. There are many types of lager; pale lager is the most widely-consumed and commercially available style of beer in the world; Pilsner, Bock, Dortmunder Export and Märzen are all styles of lager. There are also dark lagers, such as Dunkel and Schwarzbier.

History of lager brewing
While cold storage of beer, “lagering,” in caves for example, was a common practice throughout the medieval period, bottom-fermenting yeast seems to have emerged as a hybridization in the early 1400s. However, in 2011 an international team of researchers writing in the Proceedings of the National Academy of Sciences claimed to have discovered that Saccharomyces eubayanus, a yeast native to Patagonia is responsible for creating the hybrid yeast used to make lager.
 
Characteristics
The average lager in worldwide production is a pale lager in the Export or Pilsner styles. The flavor of these lighter lagers is usually mild and the producers often recommend that the beers be served refrigerated. However, the examples of lager beers produced worldwide vary greatly in flavor, color, and composition.

In colour, helles represent the lightest lager at as pale a colour as 6 EBC. The darkest are Baltic porters, which can be as dark as 400 EBC; darker German lagers are often referred to as Dunkels.
 
The organism most often associated with lager brewing is Saccharomyces pastorianus, a close relative of Saccharomyces cerevisiae.

(Oxford English Dictionary)
lager beer, n.
Pronunciation:  /ˈlɑːgə ˈbɪə(r)/ Forms:  Also simply lager.
Etymology:  < German lager-bier beer brewed for keeping, <

lager a store + bier beer.
A light beer, consumed largely in Germany and America, and to some extent in England.
1853 A. Ure Dict. Arts I. 153   Beers at present brewed in Germany.‥ 11. Wheat Lager-beer (slowly fermented).
1858 N.Y. Express June,  The German drinks his lager, and drinks it apparently in indefinite quantities.
 
lager, n.
Pronunciation:  /ˈlaːgə(r)/
Etymology:  Shortening of lager beer n.
orig. U.S.
= lager beer n. Also, a drink of this.
Lager has long been more usual than lager beer.
1855 J. E. Cooke Ellie i. i. 13   He was rotund, red and solemn—‘lager’ was written in his eyes.
1859 J. R. Bartlett Dict. Americanisms (ed. 2) (at cited word),  The German drinks his lager, and drinks it apparently in indefinite quantities.
     
1 December 1849, Public Ledger (Philadelphia, PA), pg. 2, col. 2 ad:
We would like to call the attention of the public to the new German Lager Beer Saloon, which will be opened by Mr. John A. DIVER, in Third street, above Chestnut. The very best Lager Beer we expect to be sold.
 
16 October 1850, Trenton (NJ) State Gazette, pg. 1 ad:
CHRISTIAN SARTORIUS,
LAGER BEER SALOON,
KEEPS constantly on hand that Celebrated Lager Beer from Heiner & Schmitt, Philadelphia; also the best kind of Swiss and Limburger Cheese.
 
Google Books
The Pictorial Sketch-Book of Pennsylvania
By Eli Bowen
Philadelphia, PA: Willis P. Hazard
1852
Pg. 20:
Within the suburbs of the city, scattered along the rail-road, several ” lager beer” establishments will be noticed. These breweries are all of very recent origin, and lager beer is, to many, an unknown beverage. It is a German drink,of which they are very fond, and is similar in taste and appearance to porter, but is said to have none of its deleterious qualities. It is a weak, bitter, but not unpleasant beer, containing an abundance of hops. It derives its peculiar value and flavor from storage in vaults, as the word “lager” sufficiently implies. The longer it is stored, the finer becomes its quality. The vicinity of Fairmount has lately become the fountain-head of this description of manufacture, and it is consquently a favorite resort for Germans, ...
 
3 July 1854, Albany (NY) Evening Journal, pg. 2:
Lager Bier.
From the Journal of Commerce.
The frequency with which placards bearing this inscription meet the eye, the recency of its introduction into this city, and the cabalistic character of the word themselves, may perhaps render a brief article upon this topic not uninteresting. Lager bier is a malted liquor, originally made in Bavaria, in essential properties identical with ordinary ale, which it closely resembles in appearance, though differing in taste; of much less specific gravity; weaker and retaining its foam a shorter time after being drawn, Its taste is sub-acid, pungent, and leaves in the mouth a peculiar flavor, caused by a coating of pitch which the interior of the barrels receive before being filled. The diference between the modes of brewing lager bier and ordinary ale, is indicated by the etymology of the name Lager,—meaning rest,—remaining in store; the former requiring to rest in a cool vault from four to six months before it becomes drinkable, while the latter can be used immediately after being emptied from the vats.
 
In Bavaria, the manufacture is carried on under government inspectors, the brewing period being prescribed by law, from 29th September to 23d April, the festivals of St. Nicholas and St. George. There, it is of two kinds, one of which retains its flavor only for a day or two; and the beer drinkers of Bavaria, who are very numerous, indulge so capricious and delicate a palate, that when assembled in their beer houses, they wait impatiently, if the cask in use be half empty, for a fresh one to be tapped.It is said that in well frequented houses of this kind, an ordinary cask lasts about an hour. From a German treatise, entitled Der Bier Brauer, we learn that “the attachment of the Bavarians to the beer beverage is such—a beverage descended from their remotest ancestry—that they regard the use of ardent spirits, even in moderation, as so immoral a custom, as almost to disqualify habituates of liquor drinking from respectable society.”
 
In effect, it is very moderately exhilirating, having but feeble intoxicating properties. Indeed, if it were not comparatively innocuous in its effects, the enormous quantity consumed would effect sad havoc among the drinkers—it being no unusual occurrence for an individual to drink a gallon daily, and even more. That it will ever become a favorable beverage with Young America, however, is not probable; the liking for it not being natural, but acquired. If it is drank for the gratification of the palate, Americans can readily obtain a more delicious, or to get up exuberance of spirits, it will be found entirely too low pressure for the purpose, and require too widely distended a stomach for locomotion afterwards. Its introduction has also created the necessity for a new article in glassware; for a fastidious drinker would be as reluctant to imbibe his favorite draught from any other vessel than a Lager-bier glass, as an American would dislike to drink his morning coffee from a tumbler. These glasses are of the usual shape, savethat they arewider at the base than at the brim, and have a curling ear, like the wine bottles which John Gilpin carried suspended on his belt, on the day his celebrated ride “unto the Bell of Edmonton.”
 
To gain an idea of the partiality of the German palate for this beverage, let the reader enter one of the German restaurants, and he will find that every order for “Kalbabraten mit Kartoffen: “Speck mit Eir,” &c., &c., is usually accompanied with the supplement “und ein Glas Lager-bier.” Upon them it seems to have an eminently soothing and tranquilizing effect, and under its gentle inspiration, they grow communicative and even eloquent. A young lady, whose music teacher was particularly cross and fault finding, upon complaining of his temper to a friend who was also his pupil, was laughingly offered the suggestion of providing him with a glass of Lager-Bier, on his next visit. Accordingly it was done, and repeated at every future lesson, and the pupil henceforth ever spoke highly of the amiability of her precoptor, and his assiduity instructing her. It is not our present purpose to moralize upon the use or abuse of these beverages. We are only dealing with facts; but when, as an illustration of the amount Lager-Biuer our German friends imbibe, we mention that, in the little village of Hoboken, from one hundred and fifty to one hundred and eighty barrels are consumed weekly, it shows plainly what can be done in larger places. In some of the Lager-bier gardens, where it is drank in the open air, a band of music enlivens the scene, though whether the beer is swallowed for the music or the music for the beer, we cannot clearly understand.
 
We know of but one larger brewery in New York, though the article is made to some extent in Newark; but the great bulk of what is consumed here, is brought from Philadelphia, where enormous quantities are both brewed and used. In Michigan and Illinois also, but especially Wisconsin, an immense business is carried on by lager-bier brewers, and there may be seen breweries conducted on a truly gigantic scale, and the process carried to its highest perfection.
 
In the lower counties of Pennsylvania, it is universally found, and hundreds of dealers make a handsome living by retailing it. Its price is from $4 1/2 to $5 per barrel—at present somewhat higher; and will soon be found at every locality where German customers prevail, or where German settlers have congregated together.