The common loon or great northern diver (Gavia immer) is a large member of the loon, or diver, family of birds. Breeding adults have a plumage that includes a broad black head and neck with a greenish, purplish, or bluish sheen, blackish or blackish-grey upperparts, and pure white underparts except some black on the undertail coverts and vent. Non-breeding adults are brownish with a dark neck and head marked with dark grey-brown. Their upperparts are dark brownish-grey with an unclear pattern of squares on the shoulders, and the underparts, lower face, chin, and throat are whitish. The sexes look alike, though males are significantly heavier than females. During the breeding season, loons live on lakes and other waterways in Canada, the northern United States (including Alaska), and southern parts of Greenland and Iceland. Small numbers breed on Svalbard and sporadically elsewhere in Arctic Eurasia. Common loons winter on both coasts of the US as far south as Mexico, and on the Atlantic coast of Europe.

Common loons eat a variety of animal prey including fish, crustaceans, insect larvae, molluscs, and occasionally aquatic plant life. They swallow most of their prey underwater, where it is caught, but some larger items are first brought to the surface. Loons are monogamous; that is, a single female and male often together defend a territory and may breed together for a decade or more. Both members of a pair build a large nest out of dead marsh grasses and other plants formed into a mound along the vegetated shores of lakes. A single brood is raised each year from a clutch of one or two olive-brown oval eggs with dark brown spots which are incubated for about 28 days by both parents. Fed by both parents, the chicks fledge in 70 to 77 days. The chicks are capable of diving underwater when just a few days old, and they fly to their wintering areas before ice forms in the fall.

The common loon is assessed as a species of least concern on the IUCN Red List of Endangered Species. It is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds applies. The United States Forest Service has designated the common loon a species of special status because of threats from habitat loss and toxic metal poisoning in its US range.

The common loon is the provincial bird of Ontario, and it appears on Canadian currency, including the one-dollar "loonie" coin and a previous series of $20 bills. In 1961, it was designated the state bird of Minnesota, and appears on the Minnesota State Quarter and the state Seal of Minnesota.

The adult common loon can range from 66 to 91 cm (26 to 36 in) in length with a 127 to 147 cm (4 ft 2 in to 4 ft 10 in) wingspan, slightly smaller than the similar yellow-billed loon. On average, it is about 81 cm (32 in) long and has a wingspan of 136 cm (54 in). Its weight can vary anywhere from 2.2 to 7.6 kg (4.9 to 16.8 lb). Sizes vary regionally, especially by body mass, with the smallest bodied loons on average from lower-central Canada and the Great Lakes, while westerly birds are similar or mildly larger, and loons breeding further east can appear to be significantly larger. Furthermore, males average up to nearly 27% more massive than females in some populations. Breeding loons in Maine averaged 4.65 kg (10.3 lb) in females and 5.97 kg (13.2 lb) in males, essentially the same weight as the yellow-billed loon, although the yellow-billed is still larger than in linear dimensions (especially bill length) than the Maine loons. In Ontario, 20 females averaged 4.5 kg (9.9 lb) and 20 males averaged 5.46 kg (12.0 lb). In contrast, in the Gulf of Alaska, adults of both sexes reportedly averaged 4.13 kg (9.1 lb). Adult breeding plumage consists of a broad black head and neck with greenish, purplish, or bluish sheen. It has a black bill sometimes with a pale tip, and red eyes. The neck is encircled with a characteristic black ring and has two white necklaces of eight to ten short streaks on the upper foreneck, and a noticeable collar of white, parallel lines forming a large oval on the neck-side. The central lower foreneck is pure white, and the lower neck-sides has longitudinal white lines becoming rows of small spots and black lines becoming very narrow. The upperparts are blackish or blackish grey, and each feather has small white spots on it. The upperwing is blackish and with small white spots on the non-primary coverts, whereas the underwing is paler with white coverts except the long black shaft-streaks on the axillaries. The underparts are pure white, but have some black on the undertail coverts and vent. It has a checkered black-and-white mantle and a blackish tail. The legs are pale grey on the inner half and blackish on the outer half, and the webs between the toes are flesh colored.

Adult non-breeding plumage is brownish with a dark neck and head marked with dark grey-brown. The eyes are surrounded with white, and the eyelids are pale. The bill is mostly pale grey, with a dark culmen and tip, but in early spring the tip may turn whitish. The underparts, lower face, chin, and throat are also whitish. The foreneck is whitish, usually forming wedge-shaped notch in dark neck-sides, and may sometimes reveal a shadowy trace of the neck ring or a pale collar. It has dark brownish grey upperparts with an unclear pattern of squares on the shoulders and some wing coverts spotted with white, which are usually concealed while swimming. The male and the female have similar appearances, although they exhibit sexual dimorphism in their physical dimensions with the male larger and significantly heavier than the female.

The heavy dagger-like bill is evenly tapered and greyish, sometimes having a black tip. The bill colour and angle distinguish this species from the yellow-billed loon. The neck is short and thick. The common loon swims very low in the water, with sometimes only its head held above and horizontal to the surface. It must run across the water surface to get in flight. During flight, its head is slightly lower than its body, with its feet trailing behind. It has a skeletal structure made up of a number of solid bones (this is usual for the Gaviiformes and penguins but unlike most flying birds which have bones with extensive pneumatization, hollow and filled with air to make the skeleton lighter), which adds weight but helps in diving.

A juvenile often has a dark, brownish-grey nape that may look darker than the pale-edged black feathers. It has a dark grey to black head, neck, and upperparts, with white throat, cheeks, and underparts. During the first winter, the bill shape of the young may not be as fully developed as that of the adult, and during the second winter, it much resembles the breeding adult, but with wing coverts lacking white spots.

The common loon is distinguished from the black-throated loon (G. arctica) and the red-throated loon (G. stellata) mainly by its larger size. It usually has a steeper forehead and a bulging forecrown, somewhat similar to the black-throated loon. Its bill is heavier and the back is paler than its hindneck. It is more difficult to separate from the yellow-billed loon, but its breeding plumage has more white markings on the neck and the squares on its shoulders are usually smaller; the non-breeding plumage has darker neck-sides contrasting more sharply with pale areas and bill colour.

The scaly juvenile plumage is retained until January or February of the year following hatching, when a lengthy moult of head and body feathers gives them a more adult-like appearance. Adults shed all their flight feathers simultaneously around this time, leaving them temporarily flightless, prior to gaining breeding plumage, but second-year birds delay this substantial moult until the summer. The adult winter plumage is attained between October and January by partial moult mainly of head, body and tail feathers.

With improved gene-sequencing technology, a draft genome of the common loon has assembled and identified at least 14,169 genes. 80.7% of chicken genes are found in the common loon genome. The physiological costs of deep-water diving and long distance aerial migration of loons have greatly affected loon evolution. Many identified genes are candidate genes for positive selection since the common loon-chicken split 90 million years ago. It is theorized that these candidate genes are related to haemoglobin affinity for oxygen, solute exchange, immunoglobulin function related to immune defence, nervous system development and a number of molecular pathways related to DNA metabolic function, and G-receptor pathways potentially involved in low-light visual acuity. For example, SLC48A, and SLC20A1 are candidate genes in the Gavia lineage for maintaining homeostasis due to maybe having a role in maintaining ion and pH balance.