This term is generally used to indicate unadulterated perfect bliss, got by the realization of God or the Self. In fact, one of the three basic terms used to indicate the nature of God is ānanda, the other two being sat (existence) and cit (consciousness). The second chapter of the Taittirīya Upaniṣad designated as Ānanda Vallī shows by a calculation of its own, that brahmānanda or bliss of Brahman is infinitely superior to the greatest joy that a human being ideally situated in life can ever hope to get.
The word is also used to indicate Lord Viṣṇu, a particular variety of Sanskrit metre and particular combinations of nakṣatras (stars) with weekdays, like Aśvinī being in conjunction with Sunday, and so on.
The word is also used as a suffix in the names of saṁnyāsins (monks), though it is difficult to say since when exactly the practice began. Earlier saṁnyāsins like Śaṅkara have not used it. Later ācāryas (pontiffs) of the Śṛṅgerī Monastery (for instance, Śaṅkarānanda) have started using it.