Oak apple in the context of Quercus

An oak is a hardwood tree or shrub in the genus Quercus of the beech family. They have spirally arranged leaves, often with lobed edges, and a nut called an acorn, borne within a cup. The genus is widely distributed in the Northern Hemisphere, with some 500 species, both deciduous and evergreen. Fossil oaks date back to the Middle Eocene. Molecular phylogeny shows that the genus is divided into Old World and New World clades, but many oak species hybridise freely, making the genus's history difficult to resolve.

Ecologically, oaks are keystone species in habitats from Mediterranean semi-desert to subtropical rainforest. They live in association with many kinds of fungi including truffles. Oaks support more than 950 species of caterpillar, many kinds of gall wasp which form distinctive galls (roundish woody lumps such as the oak apple), and a large number of pests and diseases. Oak leaves and acorns contain enough tannin to be toxic to cattle, but pigs are able to digest them safely. Oak timber is strong and hard, and has found many uses in construction and furniture-making. The bark was traditionally used for tanning leather. Wine barrels are made of oak; these are used for aging alcoholic beverages such as sherry and whisky, giving them a range of flavours, colours, and aromas. The spongy bark of the cork oak is used to make traditional wine bottle corks. Almost a third of oak species are threatened with extinction due to climate change, invasive pests, and habitat loss.

Apples appear in many religious traditions, often as a mystical or forbidden fruit. One of the problems identifying apples in religion, mythology and folktales is that as late as the 17th century, the word "apple" was used as a generic term for all (foreign) fruit other than berries, but including nuts. This term may have extended to plant galls such as oak apples, as they were thought to be of plant origin. When tomatoes were introduced into Europe, they were called "love apples". In one Old English work, cucumbers are called eorþæppla (lit. "earth-apples"), just as in French, Dutch, Greek, Hebrew, Afrikaans, Persian and Swiss German as well as several other German dialects, the words for potatoes mean "earth-apples". In some languages, oranges are called "golden apples" or "Chinese apples". Datura is called "thorn-apple".

At times artists co-opted the apple, whether for ironic effect or as a stock element of symbolic vocabulary. Thus, secular art as well made use of the apple as symbol of love and sexuality. It is often an attribute associated with Venus who is shown holding it.