一いち定じょう

い

ち

じょ

う

Meanings

1

dated term

Dated adverb used for emphasis, similar to 'indeed' or 'truly'. Not used in modern everyday speech.

一いち

定じょう

真しん

実じつ

で

あっ

た

。

His story was certainly true.

2

nari-adjective (archaic / formal), noun

archaic

Archaic noun/adjective meaning something fixed or inevitable. Found in classical texts; not used in modern Japanese.

必かならずN4sense 1
必ず is the everyday modern word for 'certainly/without fail', while 一定 is a dated literary adverb with the same meaning.
確たしかにN3sense 1
確かに is the standard modern adverb for 'certainly/surely', whereas 一定 is an old-fashioned equivalent used mainly in classical or emphatic speech.

一いっ定ていN3
一定 read いってい is a common modern word meaning 'fixed' or 'constant', while 一定 read いちじょう is a rare archaic reading meaning 'certainly'; they share the same kanji but differ completely in reading and meaning.

The historical derivation is uncertain; the spelling 一定 is conventionally associated with the meaning 'fixed' or 'certain'.