Butterfat

Butterfat or milkfat is the fatty portion of milk. Milk and cream are often sold according to the amount of butterfat they contain.

The fatty acids of butterfat are typically composed as follows (by mass fraction):

As shown above, the composition of fats in milk is usually discussed in terms of the fatty acids. Fatty acids do not occur as such in milk (and rarely in any food). Instead, they are incorporated into compounds called triglycerides.

In the U.S., there are federal standards for butterfat content of dairy products. Many other countries also have standards for minimum fat levels in dairy products. Commercial products generally contain the minimum legal amount of fat with any excess being removed to make cream, a valuable commodity.

...
Wikipedia