Bonsai (盆栽, "tray gardening", in Japanese) is the art of growing trees and plants, kept small by being grown in a pot and by the use of skilled pruning, formed to create an aesthetic shape and the illusion of age, although many bonsai trees are quite old and simply show their age in miniature form. The Chinese art of penjing is very similar to and is the precursor of the Japanese art of bonsai.

A bonsai is not a genetically dwarfed plant. It is any tree or shrub species actively growing but kept small by crown and root pruning. Theoretically, any species could be used, though ones with attributes such as small leaves and twigs will generally make better bonsai, helping to create the illusion of a larger tree in miniature. Properly maintained bonsai can have lifespans that might be able to reach that of their full-sized counterparts. However, bonsai require a great deal of care, and improperly maintained bonsai trees may not survive.

In the art of bonsai a sense of aesthetics, care, and patience come together. The plant, the shaping and surface of the soil and the selected container come together to express "heaven and earth in one container" as a Japanese cliché has it. Three forces come together in a good bonsai: shin-zen-bi or truth, essence and beauty.

Traditional subjects for bonsai are pine, maple, flowering apricot, japanese wisteria, juniper, flowering cherry, and larch. The plants are grown outdoors and brought in to the tokonoma at special occasions when they most evoke the current season.

The Japanese bonsai are meant to evoke the essential spirit of the plant being used: in all cases, they must look natural and never show the intervention of human hands. Chinese penjing may more literally depict images of dragons or even be guided to resemble highly intricate Chinese characters, such as 壽, "longevity", in various styles, but usually cursive

.There are many different styles of bonsai, but some are more common than others are. These include formal upright, informal upright, cascade, semi-cascade, raft and literati.

The formal upright is just as the name suggests, and is characterized by a tapering trunk and balanced branches. The informal upright is much like the formal, but may bend and curve slightly, although for aesthetic quality the tree should never lean away from the viewer.

Cascade and semi-cascade are modeled after trees that grow over water or on the sides of mountains. Semi-cascades do not lean as far downward as the cascade style.

Raft style bonsai are those that are planted on their side, and can include many other styles such as sinuous, straight-line, and group planting styles. These all give the illusion of a group of trees, but are actually the branches of a tree planted on its side.

The literati style is the hardest to define, but is seen often. The word literati is used in place of the Japanese bunjin which is a translation of the Chinese word wenjen meaning "scholars practiced in the arts". The literati style is usually characterized by a small number of branches typically placed higher up on a long, contorted trunk. Their style is inspired by the Chinese paintings of pine trees that grew in harsh climates, struggling to reach the light of the sun.

Additionally, bonsai are classed by size. There are a number of specific techniques and styles associated with mame and shito sizes, the smallest bonsai. These are often small enough to be grown in thimble-sized pots, and due to their miniscule size require special care and adhere to different design conventions.