prefix
ordinal prefix
Attaches to numerals to form ordinal numbers (e.g., 第一 = first, 第二 = second). Used in formal, academic, and everyday contexts.
第一回の会議は来週です。
The first meeting is next week.
彼はマラソンで第三位に入った。
He came in third place in the marathon.
目 is a suffix for ordinal numbers (e.g., 三つ目 = the third one), more casual and often used with native Japanese numbers, whereas 第 is a prefix used with Sino-Japanese numbers.
From Middle Chinese 第 (deiH), meaning 'order, sequence'. Used as a prefix in Japanese to form ordinal numbers.