A priest. Who performs Yagya for somebody, he is his Ritwik. According to Ved, Ritwij are the four priests - Hotaa, Adhwaryu, Brahmaa and Udgaataa who perform religious ceremonies. Their functions are ascertained by Brahmaa Jee himself. Hotaa's Karm is Shastra, Adharyu's Karm is Ijyaa, Udgaataa's Karm is Stutistom, and Brahmaa's Karm is Praayashchit (to do some difficult work as a repentance). Thus these four priests chant the four Ved in a Yagya - (1) Hotaa chants Rig Ved, (2) Adhwaryu chants Yajur Ved, (3) Udgaataa chants Saam Ved, and (4) Braahman chants Atharv Ved. When all these priests are present in a Yagya, that is called Chaaturhotra Yagya.

How Many Ritwij?
Pulastya Jee said - "There are four types of people who help in doing Yagya - Brahmaa, Udgaataa, Hotaa and Adhwaryu. Each has 3 people as his family whom they select themselves.

(1) Brahmaa has Braahmanaanchhasee, Potaa, and Aagneedhra; and this is called Brahmaa's family. These four people are good at Aanveekshikee (logic) and Ved.
(2) Udgaataa has Pratyudgaataa, Pratihartaa and Subrahmanya; and this called Udgaataa's family.
(3) Hotaa has Maitraavaruni, Achchhaavaak and Graavastut; and this is called Hotaa's family.
(4) Adhwaryu has Pratiprasthaataa, Neshtaa and Unnetaa; and they are in Adhwaryu's family.

Thus the chief learned people in Ved have told these 16 Ritwij. Brahmaa Jee himself has created 366 types of Yagya. In all of them, the same number of Ritwij have been prescribed. Some need one Sadasya (member) and 10 Cham-Saadhwaryu also apart from the above-mentioned 16 Ritwij.
[Padm Puraan, p 109]

Among these Brahmaa who is the master of Shaastra and Brahm Vidyaa, superintends and rectifies any errors and averts any ill-effects;  the Udgaataa sing hymns; and the Hotaa and the Adhwaryu perform the rites.