There are a number of senses to the meaning of this question and so there are a number of different answers.
Light is slowed down in transparent media such as air, water and glass.
This was discovered by Jean Foucault in 1850. This quantity is also referred to as c . However, this is not the end of the matter. The SI is based on very practical considerations. Definitions are adopted according to the most accurate known measurement techniques of the day and are constantly revised.