Definition: Patch Tuesday is the name given to the day each month that Microsoft releases security and other patches for their operating systems and other software. Patch Tuesday is always the second Tuesday of each month.
The most recent Patch Tuesday was on May 8, 2012. You can read more about those updates here: Patch Tuesday: May 2012.
Note: Most Windows users will experience more of a "Patch Wednesday" because they're prompted to install, or notice the installation of, the updates downloaded via Windows Update on Tuesday night or Wednesday morning.
Security related patches that are released on days other than Patch Tuesday are referred to as out-of-band security updates.
Examples:
"On one Patch Tuesday, Microsoft released fixes for over 20 security issues!"
