The Activity of Concealment
Tirodhāna ('disappearing,' 'concealment,' from tiras = beyond/hidden + dhā = to place) is the fourth of the five cosmic activities (pañca-kṛtya): sṛṣṭi (creation), sthiti (sustenance), saṃhāra (dissolution), tirodhāna (concealment), and anugraha (grace/liberation). Tirodhāna is the activity by which the Lord conceals His true nature — both from Himself in the avatāra (through the device of apparent ignorance) and from bound souls (through māyā and karma).
Concealment at Two Levels
At the cosmic level, tirodhāna is the activity by which the Lord remains hidden within His creation — the universe does not obviously display Brahman's presence, requiring the mediation of scripture, guru, and inner seeking to recognize it. At the individual level, tirodhāna is the karma and māyā that obscure the jīva's awareness of its own nature and of Brahman.
Why Concealment?
The theological question: why would a compassionate Lord conceal Himself? Sri Vaishnava theology answers: tirodhāna is necessary for the karma-dharma framework to function. If Brahman's presence were unmistakably obvious to all, the jīva could not freely choose its orientation — there would be no genuine devotion, only coerced adoration. Tirodhāna preserves the space in which genuine love and surrender become possible.
Anugraha as Resolution
Tirodhāna is always paired with anugraha (grace): the Lord conceals temporarily, then reveals through grace. The ācārya's teaching, the Āzhvār's pasurams, and the archa-avatāra are all forms of the Lord's anugraha that pierce the tirodhāna and reveal His true presence.