Kernel extension

Kernel extensions, also known as "kexts" or "legacy system extensions", are a deprecated method of modifying the kernel of Mac OS X to add low-level functionality. These are referred to as "s" in some other operating systems.

Description
Kexts filled the role of extensions previously used in classic Mac OS, but with greater stability under the Unix-based architecture of Mac OS X. After startup, problematic processes can be viewed and sometimes quit through the Activity Monitor utility.

Deprecation and replacement
Kexts were later deemed to be a security risk without a straighforward graphical user interface to manage them. The Extensions pane was introduced in OS X 10.10 (Yosemite) in 2014 to help manage these extensions. Starting with macOS High Sierra (10.13) in 2017, installation of kexts was limited to users with administrator privileges, though this did not affect pre-existing kexts during an upgrade.

Apple introduced system extensions in macOS Catalina (10.15) in 2019 as a more secure alternative that would only operate in. The Extensions pane can be used to manage both the modern system extensions as well as the legacy kexts. Starting in macOS 10.15.4, the continued use of kernel extensions would trigger a warning to the user. Support for kexts would start being phased out in macOS Big Sur except for Apple-approved enterprise developers. Any legacy kexts still installed would require user approval to be loaded upon every startup.