The person who carries out the kriya in yajnas. Agnidhra, Brahmanachhamsi, Maitra-Varuna, Hotha, Potha, Prashasta, Adhwarya, Gravastut, Brahman, Pratiharta, Subramanya, Ulgata, Pratiprasta, Neshta, Neta, Achhavaka are the 16 ritwiks. Those who conduct yajnas like Agnisnana, Agnishtoma, Pakayajna etc are conducted by ritwiks as per Manusmriti Chapter 2 verse 143.