Brahma Knowledge, by L. D. Barnett, , at sacred-texts.com
§ 3. The Upanishads. 1—The earlier Upanishads, a series of philosophic tracts of varying length and character, arose in the schools of the Brāhmaṇas, and especially were attached to the sections of the latter styled Āraṇyakas. The Hindus class together Vedas, Brāhmaṇas, Āraṇyakas, and Upanishads under the general title of "Veda" (knowledge) or "Revelation" (śruti, "hearing").
The Vedas and Brāhmaṇas are the handbooks of a crude naturalistic ritualism; but the Āraṇyakas ("Forest-books"), apparently intended
for the study of anchorites in the woods, where the more elaborate liturgies were suspended, deal more with the theory of ritual, chiefly from an allegorical point of view. Hence they lead over to the earlier Upanishads, which express a series of generally cognate metaphysical and psychological ideas, at first by allegorical interpretation of Vedic ritual and myth, later with increasing independence of method. Their relation towards Vedic ritualism was at first one of opposition; preaching the saving grace of knowledge alone, they regarded as inadequate the actual liturgies, which admittedly aimed only at worldly welfare. Later their attitude became more conciliatory, and we find them styling themselves Vedānta (first Muṇḍ. III. ii. 6, Śvet. VI. 22).
12:1 The etymology of the word Upanishad is not clear. European scholars generally regard it as from the root sad (compare Latin sedeo), so that it would mean "a sitting before," i.e. a lesson; but native tradition interprets it as "mystery," and the enigmatic formulae in which the older Upanishads sometimes convey their theories lend some support to this view (see Deussen, Philosophy of Upanishads, English translation, pp. 10 ff.).