The common swift is a medium-sized bird, superficially similar to the barn swallow or house martin but somewhat larger, though not stemming from those passerine species, being in the order Apodiformes. The resemblances between the groups are due to convergent evolution, reflecting similar contextual development. The swifts' nearest relatives are the New World hummingbirds and the Southeast Asian treeswifts.

A martlet in English heraldry is a mythical bird without feet that never roosts from the moment of its drop-birth until its death fall; martlets are proposed to be continuously on the wing. It is a compelling allegory for continuous effort, expressed in heraldic charge depicting a stylised bird similar to a swift or a house martin, without feet. It should be distinguished from the merlette of French heraldry, which is a duck-like bird with a swan-neck and chopped-off beak and legs. The Common Swift rarely lands outside breeding season, and sleeps while airborne.