It is an adverb and it means "not quite".
It is used when describing a situation in which something could be better or when you feel something is missing. "微妙" is used in the same way but it is also used when it is difficult to express your opinions clearly.
I watched the movie that my friend highly recommended, but it wasn't quite what I expected it to be.
友達のお薦めの映画を見たけど、いまいち(or微妙)だった。
友達のお薦めの映画を見たけど、いまいち(or微妙)だった。
I participated in the full marathon after a while. The result could be better but I finished 10 minutes later than I planned.
久しぶりにフルマラソンに参加した。目標より10分も遅く、結果はいまいち(or微妙)だった。
久しぶりにフルマラソンに参加した。目標より10分も遅く、結果はいまいち(or微妙)だった。
Leo is learning Japanese to take an exam soon.
- Ken : How's your Japanese study going?
日本語の勉強はどう？ - Leo : I can't say it's going well.
いまいち進んでいない(or上手くいっていない)。 - Ken : Why?
どうして？ - Leo : I'm still struggling with the basic grammar. What is the difference between the particle "は" and "が"? I don't feel like I quite understand.
基本的な文法にまだ苦戦している。助詞の「は」と「が」の違いって何？いまいち分からない(or理解できない)。
Meg and Amy are grabbing some drinks in a bar.
- Meg : Look at that guy! He's so hot.
あの人見て！かっこいいから。 - Amy : Who,the guy with the grey shirt? He isn't that handsome.
グレイのシャツを着た人？いまいち(or微妙)だよ。 - Meg : How about the one next to him?
彼の隣は？ - Amy : Hmm... neither looks very good.
んー、どっちもいまいち(or微妙)だな。