A grammatical term for a morpheme attached to the beginning of a word to modify its meaning.
"O" and "go" are Japanese prefixes.
接辞 is the broader term for affix, encompassing both prefixes and suffixes.
Compound of 接頭 (settō, 'prefixal') + 語 (go, 'word' or 'morpheme'). The term is a direct calque from Western linguistic terminology.