Among the largest species of hummingbird, it is characterized by its unusually long bill, being the only bird to have a beak longer than the rest of its body, excluding the tail.

The sword-billed hummingbird was first described as Ornismya ensifera by Auguste Boissonneau in 1839 on the basis of specimens from Santa Fé, Bogotá, Colombia.

[4] The generic and specific name ensifera is derived from the Latin words ensis (sword) and ferre (to carry) and means sword-wielder, referring to the species' large beak.

Adults are 13–14 centimetres (5.1–5.5 in) long excluding the bill and weigh 10–15 g (0.35–0.53 oz), with males being slightly larger on average than females.

[10] The sword-billed hummingbird perches with its bill angled upwards to reduce the strain of the heavy beak and improve balance.

[10] The length of the bill is so long, it also forces the sword-billed hummingbird to use its feet to groom, even though this takes longer than traditional beak methods.

The position of the flower's anthers and stigmas, along with the length of the corolla tube, make it an inaccessible food source to nearly every species except the sword-billed hummingbird.

[19] To obtain nectar, the hummingbird will stick its long bill down the tube of the corolla (both of which are almost exactly the same length), drink, and then retreat and hover for a few seconds before repeating the process.

Other species, such as insects, may be able to access the flower's nectar but do so by puncturing the base and feeding through a hole instead of the corolla tube.

If sword-billed hummingbird populations were to decline, there would most likely be a negative impact on the abundance of P. mixta flowers due to their extreme coevolution.

[10] Climate change and deforestation are the two most probable threats to the sword-billed hummingbird as they may lead to habitat loss and a decrease in food sources, especially of Passiflora mixta.

[19] The BBC's documentary series Planet Earth II depicted the sword-billed hummingbird flying through the forest in the episode "Jungles".