noun which may take the genitive case particle 'no', noun
third; third stage; third round
General ordinal use for a third stage, round, or level. Often appears in compounds such as 三次選考 or 三次募集 rather than as a very broad standalone word.
彼は三次選考まで進んだ。
He made it to the third round of selection.
noun-modifying (irregular)
tertiary; third-level
Used in classification terms where English often says 'tertiary,' especially in established compounds such as 三次産業. More technical than simply saying 'the third one.'
観光業は三次産業に含まれる。
Tourism is included in the tertiary sector.
noun-modifying (irregular)
cubic; third-order; third-degree
Mathematical use for degree or order three, as in 三次方程式 'cubic equation' and 三次関数 'cubic function.'
この三次方程式を解いてください。
Please solve this cubic equation.
授業で三次関数のグラフを描いた。
In class, we drew the graph of a cubic function.
Also means 'third,' but is the more general ordinal form before nouns; 三次 is especially used for stages, levels, orders, and technical classifications.
Means 'second,' 'secondary,' or 'second-order'; 三次 is the corresponding third-level or third-order term.
Can correspond to 'cubic' in some mathematical or measurement expressions, while 三次 specifically refers to degree or order three, as in functions and equations.
A Sino-Japanese compound of 三 'three' and 次 'order, sequence, next,' giving the sense of 'third' or 'third order.' This is a component explanation, not a claim about the deeper historical origin of the characters.