Ice cream is a frozen sweet dish, based on a custard. The most popular flavour is vanilla, followed by chocolate.
The most common defect in ice cream is the development of ice crystals in it. The answer on how to prevent this is easy: avoid any temperature fluctuations whatsoever. This, though, is far easier said than done. Ice cream has to be transported, unloaded (and how many times have you noticed crates of ice cream on loading docks in supermarket aisles, waiting to be put away.) Consumers are the other half of the problem. They put it in their grocery trolleys, wheel it about the store, transport it in their cars, and then put it in frost-free freezers, which keep themselves frost free by heating up from time to time. Each temperature change along the way causes small ice crystals, which are invisible and supposed to be there, to disappear, and larger ones to form in their place, and to get even larger with each subsequent temperature change.
It’s probably a problem that can’t be fixed. Some stabilizers are added to give ice creams a bit more tolerance for temperature chances before large ice crystals form. If the industry did find some additive to put into ice cream that made the ice crystal problem go completely away, likely consumers would protest about it and pay more for ice creams that didn’t have it in.
In the ice cream industry, the term “overrun” refers to the amount of air in it.
The legal definition of ice cream varies by country.
See also: Ice, Cream, Ice Cream Day, Ice Cream Freezer Patent Day, Chocolate Ice Cream Day, Creative Ice Cream Flavours Day, Frozen Yogurt Day, Ice Cream Soda Day, Hot Fudge Sundae Day, Peach Ice Cream Day, Soft Ice Cream Day, Strawberry Ice Cream Day, Vanilla Ice Cream Day
External resources: Enjoying Homemade Ice Cream without the Risk of Salmonella Infection (FDA)
The main ingredients in North American run-of-the-mill commercial ice cream are fat, non-fat milk solids (e.g. skim-milk powder), sugar, gelatin (or other stabilizers), egg and flavouring. In more expensive ice cream, all the fat comes from cream or milk, and the egg. In North America, almost all ice cream now comes in tub; the bricks of ice cream are now a part of their nostalgic past.
In America, to be called ice cream, the product must have 10% or more milk fat. Any less than that, and it can’t be called ice cream: it can only be called Ice Milk. In Canada, the requirement is a bit less stringent. The ice cream can be 8% milk fat if the recipe includes cocoa or syrups; otherwise, the ice cream must be 10% milk fat.
Ice Cream in the UK
In the UK, ice cream is a lot more complicated.
Second World War rationing on milk fat in the UK didn’t end until the early 1950s, so after the war, ice cream was made with vegetable fat. Thus emerged the popular myth of it being made of “frozen lard.” As well, Britain wasn’t a well-off country for decades after the war. It’s only from 1979 onwards as things started to pick up that UK consumers could start to match North American consumers. In between the war and the start of the 1980s, most households were on a “get-by” basis with income tax rates exceeding 60%, leaving little pocket money for anything but the most economical food.
Only when Haagen Dazs entered the ice cream market in 1989 did things get shaken up. Up until then, people had regarded ice cream as a junk food for kids that they weren’t willing to spend a lot of money. Haagen Dazs came along and said, it’s not for the kids, it’s for you, and that was a different story. Now, there are some very good ice creams in the UK, even some made from Cornish Clotted Cream. You can tell the quality of the ice cream you are buying by the prices. In 2004, 1 litre of Sainsbury’s bargain store brand ice cream, sold for 49p. 1/2 litre of Ben & Jerry’s cost £3.79; about 15 times more.
In the UK, some ice cream, usually the lower-end brands, are still sold in brick form. There are many terms used to describe ice cream in the UK such as Premium, Super Premium, Standard and Economy, but only the terms “ice cream” and “dairy ice cream” are legally defined:
Ice Cream (UK)
Must contain no less than 5% fat. The fat can come from anywhere (this has led to the popular myth of “frozen lard.”) Must contain no less than 2.5% milk protein (such as from skim milk powder.) Usually vegetable fat; in the past it was often hydrolysed palm kernel oil. 80% of the ice cream in the UK falls in the first category of being non milk-fat ice cream.
Dairy Ice Cream (UK)
The 5% fat content must come from milk fat; no other fat at all is permitted.
Usually contains between 10 and 14% milk fat.
Super Premium (UK)
Usually more than 14% milk fat.
At any rate, while Brits are entitled to be appalled at the state of the rest of the North American dairy industry and products, North America does have them beat on ice cream quality, at the low end at least. To get a good ice cream in the UK, make sure the package specifically says “Dairy Ice Cream.”
Frozen yoghurt or frozen custard.
1 cup ice cream = 5 oz / 150 g
Some people get headaches when they eat ice cream on a hot day. It appears to be caused by a nerve centre in your head which, when it gets suddenly very cold, feels that it has to heat your head up in a hurry, and so facilitates a lot of blood going to your head. About half the people who are prone to them report no or diminished occurrences if they eat their ice cream more slowly.
As early as the 4th century BC Roman nobles were eating flavoured ice. In 62 AD Roman Emperor Nero reputedly had slaves bring snow from the Appenine mountains, and the snow was mixed with fruit pulp and honey to make a sorbet. There is real evidence that in China, during the Tang period (618 – 907 AD), buffalo milk was mixed with flour and camphor and then frozen. Ice cream was served in 1671 at Windsor Castle for the feast of St George by King Charles II — but only those at the head table got any. During the Second World war, US B29 pilots put cans of ice cream mix in the rear gunners’ compartments — the freezing temperatures along with the vibrations made great ice cream.
Before the development of refrigeration to transport commercial ice cream, ice cream was a treat for very special occasions that required a lot of work by somebody in the household, and it was consumed immediately, not stored.
In May of 1744, Governor Thomas Bladen of Maryland served ice cream to his guests. One of the guests, William Black, wrote later in a letter: “a Dessert no less curious: Among the Rarities of which it was Compos’d, was some fine ice cream which, with the Strawberries and Milk, eat most Deliciously.” 
In 1773, Philip Lenzi announced his arrival in New York with the following newspaper advertisement: “Just arrived from London, Monsieur Lenzi, Confectioner. Makes and sells all sorts of fine French, English, Italian and German biskets, preserved fruits; also in brandy, jams, pastes, and jellies, which will be warranted for two or three years, with good care; all sorts of sugar plumbs, dragees, barley sugar, white and brown sugar candy, ice cream and fruits…”  By May of 1777, he was advertising ice cream for sale: “May be had almost every day, ice cream; likewise ice for refreshing wine, &c. ”  Some people use this to say that this was the first ice cream parlour in America, but that is a stretch: Lenzi actually sold a wide variety of sweets and confections — his store was not dedicated to ice cream, and in fact, the line quoted above appears at the very end of the advertisement listing all the other items he traded in. It is likely Lenzi was selling ice cream before this advertisement, but there is no actual documentation of that.
The choice in flavours of ice cream started to explode in the 1980s and 1990s. Cookies N’ Cream ice cream appeared in 1983. Chocolate Chip Cookie Dough ice cream appeared in 1991.
The trend of savoury ice creams started in expensive, leading edge restaurants around 1998, led by Ferran Adria in Spain, and Heston Blumenthal in the UK. Blumenthal captured the imagination of the English-speaking world with stories of his Smoked Bacon and Egg Ice Cream. In 2005, savoury ice creams started getting broader public recognition with flavours such as foie gras, marmite, ox tongue and parmesan.
Literature & Lore
“Spring and summer brought the arrival of cooling drinks. Outside Rag Fair, in Houndsditch in the East End [London] in the 1850s, a girl with a ‘horse-pail full of ice’ was selling something that looked like ‘frozen soap-suds’ in halfpenny eggcup sizes. In the same decade, ice cream first appeared, initially sold by Italian vendors, later by hokey-pokey men who were natives of Whitechapel and New Cut, with ‘Neapolitan’ ices that were rumoured to be frozen mashed turnip. [The origin of the word ‘hokey-pokey’ is uncertain, but it is probably connected to the novelty of ice cream — milk (or turnip), is ‘hocus pocus’, magically turned into a delicious treat.]” — Flanders, Judith. The Victorian City. London: Atlantic Books. 2012. Page 287.
 Theobald, Mary Miley. Some Cold, Hard Historical Facts about Good Old Ice Cream. Colonial Williamsburg Journal. Spring 2010.
 Rivington’s New York Gazetteer, November 25, 1773. Page 4.
 New-York Gazette, and Weekly Mercury, May 12, 1777. Page 3.