Hi,
it's close, the verb, it's pronounced with a Z. I thought I using it as a verb, as if I was dictating:
open (the) parenthesis, 3 plus 4, close (the) parenthesis.
...but now I just realized that I couldn't leave out the articles. So it should be an adjective... and it should be "closed". But it's not "closed parenthesis", it's "close parentheses", as far as I know. How weird. Anyway, you can also call them "left/right parenthesis".
Let's see if someone else has something to say about this.