the fifth (ordinal); indicates position number five (as in 'the fifth chapter' or 'the fifth time').
この本の第五章はとても重要だ。
The fifth chapter of this book is very important.
交響曲第五番は多くの人に知られている名曲だ。
Symphony No. 5 is a famous piece known by many.
今日は会議の第五回目にあたる。
Today corresponds to the fifth meeting.