The onion belongs to the Allium family, and has a specific species name of Allium cepa. Allium is the same family in which scientists classify lilies.
The main types of mature onions found in grocery stores include red onions, white onions and yellow onions. The yellow onion is the variety that farmers grow the most. Another type of onion is the green onion, also known as the spring onion or scallion, which is the immature form of the onion, grown for its shoots.
The onion is a nutritious vegetable. It contains zinc, potassium, calcium, iron, selenium, dietary fiber, vitamin C and vitamin B6. It contains no cholesterol, sodium or fat. It also contains quercetin, a flavanoid that helps to prevent certain diseases such as cardiovascular diseases and cancer, according to New Mexico State University's College of Agricultural, Consumer and Environmental Sciences.
Onions are best planted six weeks before the last frost and in an area that receives full sunlight. The soil onions thrive in has a pH of 5.5 to 7.0, is moderately well fertilized and moist, and well drained.
Though onions may be useful to people, they can be unsafe for animals. Cats, dogs, goats and cattle can, among other ailments, become anemic by eating onions. Anemia means they will not have enough red blood cells to be healthy.
In traditional Chinese medicine, there are two counter-balancing forces that when in balance, a person is healthy. These forces are yin and yang. Onions, including green onions, are considered yang. They may be used to bring a person who needs yang back into balance.