DelimiterFlashTime is an option for cells and notebooks which specifies how long in seconds a delimiter should flash for when its matching delimiter is entered.
DelimiterFlashTime->0 makes delimiters not flash.
A typical default setting is DelimiterFlashTime->0.3, which makes matching delimiters flash for 0.3 seconds.
Delimiters include parentheses, brackets and braces, as well as [[, ]] and (* and *), and paired special characters such as , .
If you enter an unpaired closing delimiter the standard Mathematica front end will beep.
You can use the front end menu item Check Balance to select ranges with balanced delimiters in an expressions.
You can set DelimiterFlashTime at the level of a single cell, a notebook, or the whole front end.
See The Mathematica Book: Section 2.10.11.
See also: ShowAutoStyles, SyntaxQ, ShowCursorTracker.