Compressed file used by Safari, a Web browser for Mac OS X; contains functionality that adds features to the Web browser, such as a new search bar; also includes a digital signature that verifies the extension developer; may store HTML, CSS, JavaScript, and media files; usually saved within the [user]/Library/Safari/Extensions/ directory.

SAFARIEXTZ files can be built using Extension Builder, a extension development tool included with Safari. To access Extension Builder, select Develop → Show Extension Builder from the Safari menu. When developing a Safari extension, Extension Builder first creates a folder with the extension ".safariextension." When complete with development, Extension Builder uses this folder to create a compressed package file with the ".safariextz" extension. NOTE: Apple introduced Safari extensions with Safari 5.0. Extensions are not enabled by default in 5.0 (they are enabled in Safari 5.0.1 and later). To enable them in version 5.0, you must first select the option to show the Develop menu in the menu bar (Safari → Preferences... → Advanced). Next, select the Develop → Enable Extensions option from the menu bar.