First Impression
noun
The first (第一) phenomenon is what's sealed into your head. This is your first impression of something.
だいいちいんしょう
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
第一印象はあまりいいものではなかったらしいです。
I heard that the first impression wasn’t very good.
第一印象では我の強そうな人だなぁと思っていたんですが、全然違っていましたね。
My first impression of you was a self-assertive person, but you were totally different, right?
オペレーターは、電話をかけてくださったお客様にとって、会社の 第一印象になるということを教えられました。
We were all taught that an operator is the first impression that a caller has of our company.