Indriya (literally "belonging to or agreeable to Indra") is the Sanskrit and Pali term for physical strength or ability in general, and for the senses more specifically. The term literally means "belonging to Indra," chief deity in the Rig Veda and lord of the Trāyastriṃśa heaven (also known as Śakra or Sakka in Buddhism) hence connoting supremacy, dominance and control, attested in the general meaning of "power, strength" from the Rig Veda.
In Buddhism, the term refers to multiple intrapsychic processes and is generally translated as "faculty" or, in specific contexts, as "spiritual faculty" or "controlling principle." In Buddhism, depending on the context, indriya traditionally refers to one of the following groups of faculties:
the 5 spiritual faculties
the 5 or 6 sensory faculties
the 22 phenomenological faculties
In the Pali Canon's Sutta Pitaka, indriya is frequently encountered in the context of the "five spiritual faculties" (Pali: ):
faith or conviction or belief ()
energy or persistence or perseverance (viriya)
mindfulness or memory (sati)
stillness of the mind ()
wisdom or understanding or comprehension ().
Together, this set of five faculties is one of the seven sets of qualities lauded by the Buddha as conducive to Enlightenment.
SN 48.10 is one of several discourses that characterizes these spiritual faculties in the following manner:
Faith/conviction is faith in the Buddha's awakening.
Energy/persistence refers to exertion towards the Four Right Efforts.
Mindfulness refers to focusing on the four satipatthana.
Stillness of the mind refers to achieving the four jhanas.
Wisdom/understanding refers to discerning the Four Noble Truths.
In SN 48.51, the Buddha declares that, of these five faculties, wisdom is the "chief" (agga).
In AN 6.55, the Buddha counsels a discouraged monk, Sona, to balance or "tune" his spiritual faculties as one would a musical instrument:
"... what do you think: when the strings of your [lute] were neither too taut nor too loose, but tuned to be right on pitch, was your [lute] in tune & playable?"
"Yes, lord.