Meaning of “Ni ten Go Zigen Haiyuu(2.5次元俳優)”

“Ni ten Go Zigen Haiyuu(2.5次元俳優)” means actor who play a character from Manga or Anime on stage.
“Ni ten go Zigen(2.5次元)” means 2.5 dimensions.
“Haiyuu(俳優)” means actor.

Ni Zigen(2次元) means two dimensions. So it also means “Anime or Manga” as a kind of slang.
If someone says “I have Ni Zigen Yome(二次元嫁)”, it means he has wife in Anime or Mange.
“Yome(嫁)” means wife.
Yes, I know, you think “it’s impossible”…, then you should understand like this “He fall in love with a character in anime or manga”

On the other hand, San Zigen(3次元) means three dimensions, yes it means our real world.
So if someone says “I have a San Zigen(3次元) girlfriend”, it means he has a real human girl friend.

Then, what is the “Ni ten go Zigen(2.5次元)”?
It means between 3 Zigen and 2 ZIgen.
That’s why “Ni ten go Zigen Haiyuu(2.5次元俳優)” means actor who play a character from Manga or Anime on stage. They looks kind of inter-mundane.

In Japan, there are many stage play of Anime or Manga.
The actors of them are really popular in Otaku girls.

Example Conversation

A: These days I start to be interested in real guys.
B: Oh, finally, you realize San Zigen(3次元) is also good?
A: Yeah, kind of. He is Ni ten go Zigen Haiyuu(2.5次元俳優).

A: 最近現実の男にも興味を持つようになってきたんだよね。
B: お、ついに３次元もいいって気づいた？
A: まぁそんな感じ。2.5次元俳優なんだけどさ。