In the words of Ralph Waldo Emerson, in his 1842 lecture The Transcendentalist: "The Transcendentalist adopts the whole connection of spiritual doctrine. He believes in miracle, in the perpetual openness of the human mind to new influx of light and power; he believes in inspiration, and in ecstasy. He wishes that the spiritual principle should be suffered to demonstrate itself to the end, in all possible applications to the state of man, without the admission of anything unspiritual; that is, anything positive, dogmatic, personal. Thus, the spiritual measure of inspiration is the depth of the thought, and never, who said it? And so he resists all attempts to palm other rules and measures on the spirit than its own....
"It is well known to most of my audience, that the Idealism of the present day acquired the name of Transcendental, from the use of that term by Immanuel Kant, of Konigsberg, who replied to the skeptical philosophy of Locke, which insisted that there was nothing in the intellect which was not previously in the experience of the senses, by showing that there was a very important class of ideas, or imperative forms, which did not come by experience, but through which experience was acquired; that these were intuitions of the mind itself; and he denominated them Transcendental forms. The extraordinary profoundness and precision of that man's thinking have given vogue to his nomenclature, in Europe and America, to that extent, that whatever belongs to the class of intuitive thought, is popularly called at the present day Transcendental...."
Ralph Waldo Emerson The Transcendentalist, 1842
From a recent edition of Merriam Webster's Collegiate Dictionary transcendentalism n. 1: A philosophy that emphasizes the a priori conditions of knowledge and experience or the unknowable character of ultimate reality or that emphasizes the transcendent as the fundamental reality
2: a philosophy that asserts the primacy of the spiritual and transcendental over the material and empirical
3: the quality or state of being transcendental
From a 1913 Webster's Dictionary: Tran`scen*den"tal*ism (?), n. [Cf. F. transcendantalisme , G. transcendentalismus .]
1. (Kantian Philos.) The transcending, or going beyond, empiricism, and ascertaining a priori the fundamental principles of human knowledge. As Schelling and Hegel claim to have discovered the absolute identity of the objective and subjective in human knowledge, or of things and human conceptions of them, the Kantian distinction between transcendent and transcendental ideas can have no place in their philosophy; and hence, with them, transcendentalism claims to have a true knowledge of all things, material and immaterial, human and divine, so far as the mind is capable of knowing them. And in this sense the word transcendentalism is now most used. It is also sometimes used for that which is vague and illusive in philosophy.
2. Ambitious and imaginative vagueness in thought, imagery, or diction.