Sense-organ- That source of knowledge, through which the contact of the soul (indra) is made with the external world and the sensory objects such as sound etc. are perceived by it.
indra ātmā, tasya karmamalīmasasya svayamarthān grahitumasamarthasyā'rthopalambhane yalliṅgaṃ tadindriyamityncyate.
(TaVā 1.14.1)