CyanogenMod is a custom Android firmware distribution. It is a free and open-source operating system for mobile devices. CyanogenMod offers features not found in the official Android operating systems, such as theming support, FLAC audio codec support, overclocking options (on certain devices), among others
While originally meant to be installable only on Google Nexus models of phones without requiring root privileges or unlocking the bootloader – it can now run on any phone with stock or near-stock Android software and no unlocked bootloaders are needed unless rooting is an explicit part of the installation process.
CyanogenMod supports more than 160 different device models from over 40 vendors.