When the Mac Defender malware emerged recently, Apple released a security update with up-to-the-minute malware definitions; it also began updating those definitions more regularly. OS X checks to see if your definitions are current; if not, it downloads new ones.
If you want to know how fresh your definitions are, launch Terminal, and type in the following: defaults read /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/XProtect.meta LastModification. Terminal will return a time stamp that indicates when your definitions were last updated. (The time stamp doesn’t represent the last time your Mac checked for an update, only the last time it downloaded one.)
If your definitions seem outdated, you can force OS X to check for new ones: at the Terminal command line, type sudo /usr/libexec/XProtectUpdater; when prompted, supply your administrator password. To confirm that your definitions are now more current, just run the defaults read command again.