The avocado is a tree and the fruit of that tree (Persea americana) in the flowering plant family Lauraceae. The avocado tree does not tolerate freezing temperatures, and so can be grown only in subtropical and tropical climates, where the fruit is sometimes called a pear or alligator pear.

The avocado tree is native to Central and South America. Introduced to California in the 19th century, it has become extremely successful there as a cash crop. Fallbrook, California, in San Diego County claims the title of "Avocado Capital of the World," and hosts an annual Avocado Festival.

The avocado fruit is botanically a berry. Horticultural varieties range from more or less round to egg or pear-shaped, typically the size of a temperate zone pear or larger, on the outside bright green to green-brown (or almost black) in color, and high in fat, with a large central seed or pit. Though the fruit does have a markedly higher fat content than most other vegetables, most of the fat in avocados exists as monounsaturated fat, which is relatively unproblematic for human health.

While dozens of varieties exist, two cultivars of avocados are commonly available: the Haas and the Florida. The former is the most common variety (pictured), with a dark rippled skin, and rich, creamy flesh. The Florida variety is larger and rounder, with a smooth, medium-green skin, and a less fatty, firmer flesh. These are occasionally marketed as low-calorie avocados.

The flesh is typically greenish yellow to golden yellow, if ripe turning dark soon after exposure to air. The avocado is very popular in vegetarian cuisine, making a good substitute for meats and cheeses in sandwiches because of the high fat content. The fruit is not sweet, but fatty, flavorful, and of smooth, almost creamy texture. It is used as the base for the Mexican dip known as guacamole.

The name "avocado" is from its Nahuatl name 'ahuacatl' which also meant testicles, with influence from the irrelevant but much more familiar Spanish avocado an obsolete form of 'abogado' (lawyer). In some countries of South America the avocado fruit is know as 'palta', which is a name that comes from the Quechua language. The Nahuatl ahuacatl could be compounded with others, as in ahuacamolli, meaning "avocado soup or sauce," from which the Spanish-Mexican word guacamole derives.

The avocado fruit does not ripen on the tree, but will fall off (and must be pick up) in a hard, "green" state, then it will ripen quickly on the ground, but depending of the amount of oil that it has the taste may be very different. Generally, the fruit is picked once it reaches a mature size, and will then ripen in a few days—faster if stored with other fruit such as bananas because of the influence of ethylene gas. The fruit can be left on the tree until required, rather than picked and stored, but for commercial reasons it must be picked up as soon as possible.

Barlow & Martin (2002) identify the avocado as a fruit adapted for ecological relationship with large mammals, now extinct (as for example the South American herbivorous giant ground sloths or Gomphotheres). This fruit with its mildly toxic pit, co-evolved with those extinct mammals to be swallowed whole and excreted in dung, ready to sprout. The ecological partners have disappeared, and the avocado plant has not had time to evolve an alternative seed dispersal technique.