is an expression in Japanese which may be used either as a 副詞（ふくし） (adverb), or as a 名詞（めいし） (noun). As a noun, it will carry the meaning of ‘foremost’, or ‘most important’. As an adverb, it can mean ‘first’, or ‘above anything’.
To use 第一（だいいち）
adverbially, pair it with に
. As a noun, it will usually be followed by の (when connecting nouns), or by the 助動詞（じょどうし） (auxiliary verb) だ
, when at the end of a sentence.
First of all, why was he here at that time?
This is the number one rule here.
More than money, being healthy is the most important thing.
Occasionally, まず ‘first of all’ will be paired with 第一（だいいち）
. This may seem redundant, but is like saying ‘the very first’ in English.
Firstly, let's pick the person that will be in charge of this plan.
第（だい） is a kanji that is often translated as ‘number’. However, the literal meaning is a bit closer to ‘position’, as it refers to the place that something has within a sequence of numbers. As such, 第一（だいいち） simply means ‘1st position’.