speed of light

noun phrase

Definition of speed of light

: a fundamental physical constant that is the speed at which electromagnetic radiation propagates in a vacuum and that has a value fixed by international convention of 299,792,458 meters per second symbol c

First Known Use of speed of light

1823, in the meaning defined above

Keep scrolling for more

Learn More about speed of light

Share speed of light

Statistics for speed of light

Look-up Popularity

Time Traveler for speed of light

The first known use of speed of light was in 1823

See more words from the same year

More from Merriam-Webster on speed of light

Rhyming Dictionary: Words that rhyme with speed of light

Britannica.com: Encyclopedia article about speed of light

Comments on speed of light

What made you want to look up speed of light? Please tell us where you read or heard it (including the quote, if possible).


tremendous in size, volume, or degree

Get Word of the Day daily email!

Test Your Vocabulary

Homophone Quiz

  • three-bears-two-of-them-look-like-theyre-whispering-to-a-third-bear-who-looks-chuffed-to-be-the-center-of-attention
  • In order to judge how people felt, the senator's office hired a firm to take a ______.
True or False

Test your knowledge - and maybe learn something along the way.


Test Your Knowledge - and learn some interesting things along the way.


Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!