This word compounds tong童 "child; youth; boy servant" and ji乩 "to divine" (cf. fuji扶乩 "divination; planchette writing"). Regional variants include Hokkientâng-ki童乩 and Cantonesegei-tung乩童 or san-daa神打.
A tongji or jitong is a person believed to have been chosen by a particular shen (chinese deity) or spirit as the earthly vehicle for divine expression. The Chinese differentiate a wu巫 "shaman; healer; spirit medium" who gains control of forces in the spirit world versus a tongji who appears to be entirely under the control of forces in the spirit world.
Andersen, Poul. 2008. "Tâng-ki (or jitong) 童乩 (or 乩童) spirit-medium", in The Encyclopedia of Taoism, ed. by Fabrizio Pregadio, pp. 964-966. Routledge.
Elliott, Alan J. A. 1955. Chinese Spirit Medium Cults in Singapore. Monographs on Social Anthropology, No. 14. Department of Anthropology, London School of Economics and Political Science.