CPU Frequency, or CPU Frequency Scaling. This is implemented in the Linux kernel, and in versions since 3.4 use an ondemand governor by default, which saves power by using as much as is needed.

CPU frequency scaling enables the operating system to scale the CPU frequency up or down in order to save power. CPU frequencies can be scaled automatically depending on the system load, in response to ACPI events, or manually by userspace programs.

CPU frequency scaling is implemented in Linux kernel, the infrastructure is called cpufreq. Since kernel 3.4 the necessary modules are loaded automatically and the recommended ondemand governor is enabled by default. However, userspace tools like cpupower, acpid, Laptop Mode Tools, or GUI tools provided for your desktop environment, may still be used for advanced configuration.

Source: https://wiki.archlinux.org/index.php/CPU_Frequency_Scaling