It is a noun and adverb that has three different meanings.
First, it means “generally“ and can be used to express the general cases/most cases of something.
Second, it means “roughly“ and can be used to make a rough estimation or to talk about the main point of something.
Third, it means “in the first place“ and can be used to mention what should have been done at the beginning of a situation.
- Ken: 会議の前に、(上司に)資料をレビューしてほしいんだけど、何時頃来るんだろう?
I want my boss to review my documents before the meeting. When do you think he will arrive at the office? - Leo: だいたい(いつも)、九時半頃出社するよ。
He usually gets to work at half past 9. - Ken: えっ?クライアントは、だいたい10時頃に来るって。それじゃぁ、資料を確認する時間は、たった30分しかないね。
Really? Our clients are supposed to come here around 10. That leaves us with only 30 minutes to go over the documents. - Leo: だいたいなんで、昨日やらなかったんだよ?会議が早朝から始まるって、知っていただろう?
Why didn’t you finish it yesterday in the first place. You knew that the meeting would start early in the morning, didn’t you?
- Meg: もう、全進捗資料を見た?
Have you already gone through every progress report? - Amy: うん。だいたいのプロジェクトが順調に進んでいるみたいだね。
Yes. Most of the projects seem to be on schedule. - Meg: でも、ABCプロジェクトはだいたい一週間遅延している。来週、遅れを取り戻せると思う?
But ABC project is about a week behind schedule. Do you think it will catch up with the schedule next week? - Amy: んー、遅れている原因は、だいたい分かったけど、直接(会って)詳細を確認したいな。
Hmm, I roughly understand why it is running late, but I want to have a quick meeting with them to go over the details.
- Meg: 夏は終わったばかりだと思っていたけど、冬が近づいているみたい。
I thought that summer was just over, but it seems like the winter is approaching. - Tom: もう10月だからね。少しずつ寒くなっていくね。
It’s October already. It’s getting colder little by little. - Meg: もう肌寒いよ。
It's already a little chilly. - Tom: だいたいなんで、この寒いのに半袖なわけ?
Anyway why are you wearing sleeveless in this cold?