adverb
certainly; veritably; to be sure
Dated adverb used for emphasis, similar to 'indeed' or 'truly'. Not used in modern everyday speech.
彼の話は一定真実であった。
His story was certainly true.
nari-adjective (archaic / formal), noun
Archaic noun/adjective meaning something fixed or inevitable. Found in classical texts; not used in modern Japanese.
一定の運命に従うほかなかった。
There was no choice but to follow a certain fate.