No English word fits. Gods, spirits, deities and divinities each import assumptions that do not hold.
Motoori Norinaga, the great eighteenth-century scholar of the native tradition, gave the definition still most often quoted: kami are the deities of heaven and earth and the spirits residing in their shrines, and also human beings, birds, beasts, trees, seas, mountains and anything else whatever which possesses extraordinary and preeminent qualities and inspires awe. He added, pointedly, that this includes things that are evil or frightening, since these too are awe-inspiring.
So the category takes in Amaterasu, the sun goddess from whom the imperial line is traced, and Susanoo, her disruptive brother; it takes in Mount Fuji, particular waterfalls, ancient trees marked with a straw rope, and unusual rocks; it takes in ancestors, emperors, and figures such as the scholar Sugawara no Michizane, enshrined after his death and now petitioned by students before examinations. New kami continue to be recognised. The traditional expression yaoyorozu no kami, eight million kami, means uncountably many rather than a figure.
Three consequences follow that a reader coming from a monotheistic background should register. Kami are not omnipotent and not omniscient; they have limited domains and can be neglected, offended or pleased. They are not necessarily benevolent; a kami with a violent aspect must be pacified, and enshrinement is sometimes precisely a means of settling a dangerous spirit. And they are not outside the world; there is no creation from nothing, the founding myths describe the islands and the kami being generated together, and the problem of evil in the Western form does not arise, because no single omnipotent benevolent will is being held responsible for everything.