Antarātmā (Antaratma; the inner self) is one of the Aṣṭottara Saharsanāma (Ashtottara Sahasranama; Thousand and eight names) of God Śiva (Siva). In Upaniṣad (Upanishad), it is said that the Supreme Self resides in the shape of a big finger, in the hearts of every living being as Jīvātmā (Jivatma; the soul of living beings)–
aṅguṣṭhomātraḥ puruṣo’ntarātmā sadā janānāṃ hṛdaye sanniviṣṭaḥ.
God Śiva (Siva)is that Supreme Self who resides as the inner self or Jīvātmā in the living beings — so He is known as Antaratmā.