Definition - What does Sabda mean?
Sabda (or shabda) is the Sanskrit word for "speech sound." It sometimes refers generally to electromagnetic waves or vibratory energy. To grammarians, sabda is a unit of language or speech. In Hindu and yogic philosophy, it describes verbal testimony as a way of acquiring knowledge of the Absolute Reality, or Brahman.
In Vedic literature, Shabda Bramhan is said to be omnipresent through the sound of Om (or Aum). Sabda is, therefore, a word expressing a particular meaning manifested by sound. Mantras in Vedic literature depict the spiritual effects of sabda.
Yogapedia explains Sabda
In some schools of yoga, sabda is an esoteric concept of the sound current that vibrates throughout all creation. The sound can be heard internally or externally through speech, mantras or music. Sabda is referred to as the inner sound, the audible life stream or the word. It is also believed to be the essence of God.
In the Upanishads, sabda first appears in an explanation of the two kinds of Brahman: Shabda Brahman (with sound) and Ashabda Brahman (without sound).
Surat Sabda yoga is a spiritual practice in the Sant Mat tradition (meditation on inner light and sound) and other related traditions. The goal of this type of yoga is union of the self with the Absolute/Supreme Being, or realization of the true Self.