The lesser scaup (Aythya affinis) is a small North American diving duck that migrates south as far as Central America in winter. It is colloquially known as the little bluebill or broadbill because of its distinctive blue bill. The origin of the name scaup may stem from the bird's preference for feeding on scalp—the Scottish word for clams, oysters, and mussels; however, some credit it to the female's discordant scaup call as the name's source. It is apparently a very close relative of the Holarctic greater scaup or "bluebill" (A. marila), with which it forms a superspecies. The scientific name is derived from Ancient Greek and Latin, with aithuia being an Ancient Greek term for an unidentified seabird, used by Hesychius and Aristotle, among other authors, and affinis, Latin for "related to", from the lesser scaup's resemblance to the greater scaup.

Adults are 38–48 cm (15–19 in) long, 41.7–43 cm (16.4–16.9 in) on average. The species can weigh 454–1,089 g (1.001–2.401 lb); males weigh 820 g (1.81 lb) on average while females weigh noticeably less, at 730 g (1.61 lb) on average. Wing lengths (not wingspans) are about 7.5–7.9 in (19–20 cm) in males and 7.3–7.8 in (19–20 cm) in females. The tarsus is about 1.4–1.5 in (3.6–3.8 cm) long, and the bill is 1.4–1.7 in (3.6–4.3 cm). The wingspan is 68–78 cm (27–31 in).

The adult males (drakes) in definitive basic (breeding) plumage have a black, iridescent head and a small tuft at the hindcrown, a black breast, a whitish-grey back, wings with darker vermiculations and black outer and greyish-brown inner primary remiges. The underparts are white with some olive vermiculations on the flanks, and the rectrices and tail coverts are black. Adult females (hens) have a white band at the base of the bill and often a lighter ear region, and are otherwise dark brown all over, shading to white on the mid-belly. Drakes in eclipse plumage look similar, but with a very dark head and breast, little or no white on the head and usually some greyish vermiculations on the wings. Immature birds resemble the adult females, but are duller and have hardly any white at the bill base. Both sexes have white secondary remiges, a blue-grey bill with a smallblack "nail" at the tip and grey feet; the drakes have a bright yellow iris, while that of females is orange to yellow varying with age and that of immatures is brown. Downy hatchlings look much like those of related species, with dark brown upperparts and pale buff underparts, chin, supercilium and back spots.

These birds are not very vocal, at least compared to dabbling ducks. Hens give the namesake discordant scaup, scaup call; in courtship drakes produce weak whistles. Hens vocalize more often than those of the greater scaup—particularly during flight—but their call is weaker, a guttural brrtt, brrtt.

Lesser scaup are often hard to distinguish from the greater scaup when direct comparison is not possible, but in North America a large scaup flock will often have both species present. Females, juveniles and drakes in eclipse plumage are hard to identify; there is considerable overlap in length between the two species, but greater scaup are usually noticeably more bulky. Lesser scaup females and immatures tend to have less white around the bill, but this too varies considerably between individual birds. Lesser Scaup generally have a smaller and straighter bill, with a relatively narrow profile, while Greater Scaup have a wider, more spatulate profile toward the tip of the bill. The dark nail tip averages wider on Greater Scaup. In flight, the most tell-tale sign is the white secondary remiges, whereas in the greater scaup the white extends on the primary remiges also, i.e. far towards the wingtip.

Lesser scaup drakes in nuptial plumage are often said to be recognizable by the purple instead of green sheen of the head and a darker back. But this is unreliable because it varies according to light conditions, and these birds are often too far away from the observer to make out any sheen at all. The shape of the head is a crucial difference, but only when birds are at rest and not actively diving. In the greater scaup drake, the forehead is usually quite massive (especially in North American and East Asian populations), whereas the nape presents a smooth shallow curve and may appear almost straightly sloping. The lesser scaup drake presents the opposite shape, with a less bulging forehead and a nape that looks strongly curved or even angular due to the small crest. When the birds raise their heads, these differences are most easy to spot, and after observing the two species in direct comparison it usually becomes easy to recognize. In fact, in basic plumage the lesser scaup drake may appear identical in shape and size to a drake of the ring-necked duck (A. collaris); the black back and wings of that species are hard to confuse with the light ones of the lesser scaup male though.

Particularly in the case of vagrant birds in Europe, the identification is complicated by similar-looking Aythya hybrids. Except for hybrids between the two scaup species, the most reliable mark is the black bill-tip of hybrids, whereas in the scaups only the very point ("nail") of the bill is black. This is even recognizable at considerable range, as the scaups' bills appear uniformly grey from a distance, whereas those of hybrids look two-colored. European hybrids typically involve the tufted duck (A. fuligula), yielding offspring that have a small nape crest unlike any European Aythya species. Female and immature hybrids typically lack the white bill base, except in those between lesser scaup and ring-necked duck, where the white extends to the eye region. But especially with juveniles, the bi-colored bill of hybrids is most diagnostic. Hybrid combinations that are known from the wild and resemble the lesser scaup are:

In theory, each and every Aythya species is able to produce potentially fertile hybrids with any other, though due to their different ranges and behavioral cues given during courtship most of these hybrids are only known from birds kept in captivity without conspecific mates.