A milkshake (sometimes simply called a shake) is a sweet beverage made by blending milk, ice cream, and flavorings or sweeteners such as butterscotch, caramel sauce, chocolate syrup, or fruit syrup. It may also be made using a base made from non-dairy products, including plant milks such as almond milk, coconut milk, or soy milk. Dry ingredients such as whole fruit, nuts, seeds, candy, or cookies are sometimes incorporated.

Milkshakes originated in the United States around the turn of the 20th century and gained widespread popularity after the introduction of electric blenders in the following decades. During this period, ice cream shops became popular social gathering spots for young people, and milkshakes came to symbolize the simplicity and innocence of youth culture.

Full-service restaurants, ice cream shops, soda fountains, and diners usually prepare the shake in a milkshake machine. At home, a blender is more commonly used. Milkshakes can also be mixed by hand using a spoon. Milkshakes may be made from any flavor of ice cream; additional flavorings, such as chocolate syrup, malt syrup, or malted milk powder, are often added prior to mixing.

Many fast food outlets do not make shakes from the individual ingredients; rather, they use automatic milkshake machines which freeze and serve a pre-made milkshake mixture consisting of milk, a sweetened flavoring agent, and a thickening agent. These are similar to soft-serve ice cream machines, but they keep the shake at a drinkable consistency.

When the term milkshake was first used in print in 1885, a milkshake was an alcoholic whiskey drink that has been described as a "sturdy, healthful eggnog type of drink, with eggs, whiskey, etc., served as a tonic as well as a treat". However, by 1900, the term referred to "wholesome drinks made with chocolate, strawberry, or vanilla syrups". By the "early 1900s people were asking for the new treat, often with ice cream". By the 1930s, milkshakes were a popular drink at malt shops, which were the "typical soda fountain of the period".

The history of the electric blender, malted milk drinks, and milkshakes is interconnected. Before the widespread availability of electric blenders, milkshake-type drinks were more like eggnog, or they were a hand-shaken mixture of crushed ice and milk, sugar, and flavorings. Hamilton Beach introduced its Cyclone Drink Mixer in 1911, and it was widely used in soda fountains.

The Hamilton Beach design, with the motor on top, remains the most common kind of milkshake machine. In 1922, Steven Poplawski invented the bottom-motor blender, which is sometimes used for making milkshakes. With the invention of the blender, milkshakes began to take their modern, whipped, aerated, and frothy form.

The use of malted milk powder in milkshakes was popularized in the US by the Chicago drugstore chain Walgreens. Malted milk powder – a mixture of evaporated milk, malted barley, and wheat flour – was invented by William Horlick in 1897 for use as an easily digested restorative health drink for disabled people and children, and as an infant's food. However, healthy people soon began drinking beverages made with malted milk simply for the taste, and malted milk beverages containing milk, chocolate syrup, and malt powder became a standard offering at soda fountains. In 1922, Walgreens employee Ivar "Pop" Coulson made a milkshake by adding two scoops of vanilla ice cream to the standard malted milk drink recipe. This item, under the name "Horlick's Malted Milk", was featured by the Walgreen drugstore chain as part of a chocolate milkshake, which itself became known as a "malted" or "malt" and became one of the most popular soda-fountain drinks.

The automation of milkshakes developed in the 1930s, after the invention of freon-cooled refrigerators provided a safe, reliable way of automatically making and dispensing ice cream. In 1936, inventor Earl Prince used the basic concept behind the Freon-cooled automated ice cream machine to develop the Multimixer, a "five-spindled mixer that could produce five milkshakes at once, all automatically, and dispense them at the pull of a lever into awaiting paper cups".

Newspaper articles from the late 1930s suggest the term 'frosted' was sometimes used to refer to milkshakes, particularly those made with ice cream. In 1937, the Denton Journal in Maryland stated that "For a 'frosted' shake, add a dash of your favorite ice cream." In 1939, the Mansfield News in Ohio stated that "A frosted beverage, in the vernacular, is something good to which ice cream has been added. Example par excellence is frosted coffee –that hot, tasty beverage made chilly with ice and frosty with ice cream."