1.
first; number one; the first
The ordinal number meaning first in a sequence or ranking.
第一印象が大切だ。
First impressions are important.
まず第一章から読み始めよう。
Let's start reading from the first chapter.
彼女はマラソン大会で第一位を獲得し、家族みんなが喜んだ。
She won first place in the marathon, and her whole family was delighted.
2.
foremost; top priority; most important
Used to indicate that something is the most important or should take priority above all else.
安全第一で作業してください。
Please work with safety first.
健康が第一だから、無理をしないでね。
Health comes first, so don't push yourself too hard.
この会社は顧客満足を第一に考え、サービスの改善に力を入れている。
This company puts customer satisfaction first and is committed to improving its services.
第一 functions both as an ordinal meaning "the first" in a sequence and as an expression of top priority. In compounds it marks order, while after a noun it conveys that something should come before all else.
COMMON COMPOUNDS:
- 第一印象 — first impression
- 第一歩 — first step
- 第一位 — first place
PATTERN: ～第一
Placing 第一 after a noun means "X first" or "X is top priority":
- 安全第一 — safety first
- 健康第一 — health first
- お客様第一 — customer first
RELATED WORDS:
- 第二 — second
- 第三 — third
- 一番 — number one; most (more colloquial)