光速，意指光在真空中的速度，是一个物理常数，一般记作c，精确值为299,792,458 米每秒（≈3.00×10 m/s）。这一数值之所以是精确值，是因为米的定义就是基于光速和国际时间标准上的。根据狭义相对论，宇宙中所有物质和消息的运动和传播速度都不能超过c。光速也是所有无质量粒子及对应的场波动（包括电磁辐射和引力波等）在真空中运行的速度。这一速度独立于射源运动以及观测者所身处的惯性参考系。在相对论中，c起到把时间和空间联系起来的作用，并且出现在广为人知的质能等价公式中：E = mc。

The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its precise value is 7008299792458000000♠299792458 metres per second (approximately 7008300000000000000♠3.00×10 m/s), since the length of the metre is defined from this constant and the international standard for time. According to special relativity, c is the maximum speed at which all matter and hence information in the universe can travel. It is the speed at which all massless particles and changes of the associated fields (including electromagnetic radiation such as light and gravitational waves) travel in vacuum. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. In the theory of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence E = mc.