What does the "c" in "E=mc²" represent?

More Questions:

In "The Twelve Days of Christmas', what did my true love send to me on the fifth day?

With which Major League team did Randy Johnson make his 1988 debut?

In the game "Battleship', how many hits does it take to sink an aircraft carrier?

Which Canadian province is the setting for the novel "Anne of Green Gables"?

In what direction do most cyclones whirl in the Southern Hemisphere?

What flying insects are classified in two main families, "hesperioidea" and "papilionoidea"?

What is 11 squared?

What is inflammation of the gums called?

In the TV show "Growing Pains', what was Boner's dad's name?

Dr. Brown's puppy in "Back to the Future Part II" was named after what astronomer?

Who was the host of TV’s “Unsolved Mysteries”?

Whose US presidential campaign was targeted by the famous "Daisy Girl" negative TV ad?

Which physical force is the only one that is always attractive, never repulsive?

What year did Beethoven finish composing Symphony No. 9?

Which European city is regarded as the watch making capital of the world?