It is an adverb.
The first meaning is “much(better)“. It is used to describe situations in which the amount/degree of something is larger than the one of something else.
The second meaning is “the whole time“. It is used to express the entire period of time since something began.
Meg and Amy are talking at the office. Meg is thinking about whether to go running after work.
- Meg: I’d like to run at the park this evening. Do you think it is going to rain later?
今夜公園で走りたいな。雨(が)降ると思う？ - Amy: Yes. It's been nothing but cloudy lately. And it often rains in the evening.
うん。最近ずっと曇っているし、夜はよく雨が降るよね。 - Meg: I brought these new shoes here. I don’t want to get them dirty right away.
新しい靴を持って来たけど、汚したくないな。 - Amy: Show me. Look nice! They are much lighter than they look.
見せて。素敵！見た目より、ずっと軽いね。
Ken and Leo are walking toward the beach after they parked their car.
- Ken:How long does it take to get there by foot?
(徒歩で)どれくらいかかる？ - Leo: Well, we need to keep going straight down this street so I think it would take 10 more minutes.
んー、この道をずーっと、真っすぐに行かないといけないから、後10分くらいかかるよ。 - Ken: Are you serious? That's so far away!
マジ？ずっと先じゃん。
Meg is talking about her plan to transfer abroad.
- Amy: Why didn’t you tell me about your transfer to Hong Kong, huh? I’ll miss you.
なんで、香港に転職する事を教えてくれなかったの？寂しくなるよ。 - Meg: Oh, It’s still a long way off. As long as the COVID-19 pandemic is not over, I won’t be able to go there.
あぁ、まだずっと先の話だよ。コロナ(の問題)が終わらない限り、行けないよ。 - Amy: I see, but I didn’t know that you’ve always wanted to work abroad.
そっか。でも、メグがずっと海外で働きたかったなんて知らなかったよ。 - Meg: It’s too embarrassing, so I couldn’t say it. Other applicants for the transfer speak English much better than me.
恥ずかしくて、言えなかったよ。他の転勤希望者の方が、ずっと英語が上手だから。
Tom is proposing to his girlfriend.
- Tom: Do you know how long it’s been since we met?
付き合って、どれくらい経つか知っている？ - Meg: I guess it’s around three years, huh?
3年くらいでしょ？ - Tom: Right. I want to be with you forever and ever. Will you marry me?
そうだね。ずっと僕と一緒にいてくれる？結婚しよう。 - Meg: Yes, I will.
はい、お願いします。