A household god is an entity worshipped in a particular household, thereby offering specific protection to the family living there. In the old polytheistic days, these were deities or aspects of them in their own right. In modern times, the term "household god" has also come to mean a person or an object that is much honoured and revered in the household. Household gods are also called house gods, but a house with a household god is rarely called a House of God.
The worship of household gods stems from the animistic belief that there are spirits all around us. A natural effect of this is that some spirits reside in a house and, if appeased, will protect it. In almost all cultures that believed in spirits, the household gods existed. I will give a few examples.
The Romans had a whole system of household gods which they worshipped with great zeal. A shrine devoted to the household gods was usually placed in the atrium, and a statue or a fresco of one of them in the entrance to the house. The gods were offered sacrifices regularly, both food and drink to keep them happy, and also symbolical pieces of the household members, such as a young man's first shaving.
There were two major groups; the Lares, which were guardians of the house and the land, and the Penates, which protected the food stores. But there was more to be worshipped. The Genius, too, were hailed in every house as protectors of the bloodline. In addition, there were gods assigned to specific parts of the house: Forculus protected doors, Limentinus presided over thresholds, and the goddess Cardea looked after door hinges. Priapus was another god prominent in the household, he symbolised bounty and fertility.
House gods took on different shapes. In Lithuanian and Prussian beliefs, the Aitvaras took on the spirit of a black cat or cock (the animal) which lived behind the hearth and brought fortune on the house. The scary Bes in Egyptian mythology was a dwarven household god who frightened away evil spirits by his appearance, especially during childbirth.
Another place where the household god was important during this time was in Samoa, where the father of the woman giving birth or her husband prayed to it all the while. To give thanks for keeping the mother and child safe and sound, the god could then be offered a canoe or even a house devoted to his priests. To show their respect to the helpful god, the child was named after him.
Among the Tharu people in Nepal, housegods are still worshipped and offered sacrifice. They are given milk and silk, but are also fond of blood. Often a chicken or a pigeon is slaughtered for the house god, or the master of the household cuts himself to give it of his blood.
While worshipping a household god may seem a bit uncivilised and antiquated, it is still done among people who consider themselves modern. In many Catholic homes, the Virgin Mary is revered in the form of a statue and thought to look after the household. In still more secular homes, the TV or the telephone are given such a prominent place that they can be considered household gods.