Used by medieval European mystics, such as the Fire-philosophers, Rosicrucians, and Qabbalists, to signify those classes of ethereal beings evolved in and born of the four elements or kingdomAs of nature. Ordinarily they are spoken of as existing in four classes corresponding to the four popular elements air, fire, water, and earth; but theosophy describes these kingdoms of nature as seven or even ten in number: four of the material or quasi-material range, and three (or six) of highly ethereal and even quasi-spiritual substance. They are often described as nature spirits or sprites. 
More strictly, the word is confined to those beings who are beginning their evolutionary growth, who have developed in their constitution but one of the four elements -- that one from which they were born -- and who are therefore in the elemental state of growth. It is a generalizing term for all beings evolutionally below the minerals. Nevertheless, by extension of meaning, the mineral, vegetable, and animal kingdoms are often referred to as families of elemental beings, though in more advanced stages. An elemental, therefore, is a being who has entered our, or any other, universe on its lowest plane or world. 
There are three kingdoms of the elementals below the mineral kingdom, each of which has seven (or ten) subdivisions, and every entity high or low has passed through this stage at some time in its career. There are four commonly recognized great classes of these unevolved beings, called by the medieval European mystics gnomes, undines, sylphs, and salamanders -- elementals respectively of earth, water, air, and fire. These elementals are not only the inhabitants of and born from the respective elements, but really are the elements themselves. They are from one viewpoint simply nature forces, tools of the higher intelligences, and actually perform all the physical work of the world. 
From another point of view they may be looked upon as life-atoms in different stages of evolutionary growth; and being in various degrees of evolution they are variously spiritual, ethereal, astral, or material, running through vast ranges on all these planes. Thus they exist everywhere: in the air we breathe, the food we eat, and all the tissues of physical nature. Through their agency we perform all our bodily or mental activities. 
The three kingdoms of elementals actually build and form every new planet or world, beginning in serial order with the lowest of the three kingdoms, preparing the globe for the advent of the mineral kingdom, to be followed in turn by the vegetable and higher kingdoms in regular succession. The elementals are not only the matters of nature, but when acting together and used by higher intelligences become the forces or energies of nature, such as electricity, magnetism, light, vitality, etc. Unconsciously, human and other beings use them in the carrying on of all their bodily functions. For example, our bodies cohere through the automatic aid of the elementals of earth; and the elementals of fire give us our bodily heat. The four kingdoms of elementals, existing in the four elements, are also known under the general designation of fairies and fays in the myths, fables, traditions, and poetry of all nations, ancient and modern. Their names are legion: peris, devs, jinn, sylvans, satyrs, fauns, elves, dwarfs, trolls, nixies, kobolds, brownies, banshees, leprechauns, pixies, moss-people, good people, good neighbors, wild women, men of peace, white ladies, and many more. They have been seen, feared, blessed, banned, and invoked in every quarter of the globe in every age. 
These elementals are the principal nature forces used by the disimbodied human dead, very real but never visible "shells" mistaken for spirits at seances, and are the producers of all the phenomena except the purely subjective. They may be described as centers of force having instinctive desires but no consciousness as we understand it. Hence their acts may be what we humans call good or bad, indifferently. They have astral forms which partake, to a distinguishing degree, of the element to which they belong and also of the universally encompassing ether. They are a combination of sublimated matter and a purely rudimental mind. Some remain throughout several cycles relatively unchanging, so far as radical change goes, but still have no separate individuality, and usually acting collectively, so to speak. Others, of certain elements and species, change under a fixed law which Qabbalists explain. The most solid of their bodies are ordinarily just immaterial enough to escape perception by our physical eyesight, but not so unsubstantial that they cannot be perfectly recognized by the inner or clairvoyant vision. They not only exist and can all live in ether, but can handle and direct it for the production of physical effects, as readily as we can compress air or water for the same purpose by pneumatic and hydraulic apparatus; in which occupation they are readily helped by the human elementaries or astral shells.
More than this, they can so condense the ether as to make for themselves tangible bodies which, by their Protean powers, they can cause to assume such likeness as the elementals themselves are at the time impressed to assume, this being caused by their taking automatically as their models the portraits they find stamped in the memory of a person or persons present at a seance. It is not necessary that the sitter should be thinking at the moment of the one represented: the image may have faded many years before. The mind receives indelible impressions even from chance acquaintances. As a few seconds' exposure of the sensitized photographic plate is all that is requisite to preserve indefinitely the image of the sitter, so is it in incomparably greater degree with the mind. Unable to invent anything or to produce anything of itself, the elemental automatically reflects stamped impressions in the memory of human beings to its very depths; hence the nervous exhaustion and mental oppression of certain sensitive natures at spiritualistic circles.The elemental will bring to light long-forgotten remembrances of the past: forms, images, even familiar sentences, long since faded from memory, but vividly preserved on the astral tablets of the imperishable book of life. The elementals are very imitative, having neither developed will nor intelligence of their own which they self-consciously use, and hence tend automatically to copy forms in all the higher kingdoms. They have therefore many shapes or bodies, some of the more advanced taking even a quasi-human form. 
Some of the elementals are said to be friendly, others unfriendly, to humanity not because of any deliberate intent on their part, but simply because mankind happens to be in such evolutionary position that it is affected one way or the other by them. Also, as different people contain in their constitution a preponderance of one of the elements over the other, they are more sensitive to the elementals of their predominating element.