TITLE: What does .9 with a line above the 9 mean?
QUESTION [26 upvotes]: What does this mean?
$$\Large.\overline9 $$
I've never seen this notation before.

REPLY [3 votes]: This symbol is called a vinculum or a overline.
The number(s) under this symbol are repeated indefinitely.
For your question:
$0.\overline{9} = 0.99999999999999...$
It could also be used like:
$2.52\overline{346} = 2.52346346346346...$