Achamana

Āchamanam (Sanskrit: आचमनम्, ) is a purification ritual performed at the beginning of more complex religious ceremonies in the Hinduism, which is believed to cure all physical and mental impurities.

Types of Āchamanam
There are three types of Āchamanam, namely, Śrautācamanam (Sanskrit: श्रौताचमनम्), Smṛtyācamanam (Sanskrit: स्मृत्याचमनम्) and Purāṇācamanam (Sanskrit: पुराणाचमनम्).

"Achamana is of three types - Śrautā, Smārta (as directed by Smritis) and Paurāṇā (as per Puranas). There Śrautācamanam is directly instructed by Śruti (i.e. Vedas), Smārtācamanam is formulated by dharmasūtras (i.e. Smṛti). Sippping (of water) thrice with the (24) names (of Vishnu) starting with keśava is called Paurāṇācamanam"

However, in the Sandhyavandana ritual, there exists fourth version of āchamana, known as mantrācamana.

Śrautācamanam
In śrautācamanam, water is sipped three times and is accompanied by the recitation of the three padas of the Gayatri in succession. Then, 21 parts of the body are touched while the 21 mantras are recited: the nine Abliṅgas, the seven Vyahritis (the names of the seven worlds, preceded by the sacred pranava Om), and the 3 padas of Gayatri siras. It is defined in ṣatkarma candrikā as

Smṛtyācamanam
The Smṛtyācamanam is an abridged version of the śrautācamanam. Water is sipped three times, accompanied by the ritual utterance svāhā. Then, 9 parts of the body are touched, accompanied by recitation of the specific sutras that instruct the version of the achamana in question.

Purāṇācamanam
Purāṇācamanam is done with the 24 names of Vishnu starting with keśava, etc. The water is poured on the right hand palm, which made as gokurna sipped thrice with the first three names. Both hands are washed with the next two names. For the remaining 19 names different parts of body are ritually cleansed.