I saw this common usage error again just the other day in a published book. Often, writers will use the word “lightening” when they really mean “lightning.”
The word “lightening,” pronounced with three syllables, is the present participle form of “to lighten,” which means to make something lighter, such as light, color, or a burden:
The sky is lightening as dawn approaches.
As Jim ate the apples he had picked in the orchard, he was lightening the bucket of fruit he carried back to his car.
“Lightning,” two syllables, is the discharge of electricity that comes out of the sky during stormy weather. It can also be used to describe something that moves like lightning:
Derrick heard thunder and saw lightning.
Did it lightning during the storm last night?
The aerial assault struck with lightning speed.
Now, I did find this definition of “lightening” at Merriam-Webster Online: “to give out flashes of lightning.” However, this is a definition for an intransitive verb, not a noun. Since “lightning” can also be used as an intransitive verb with a similar definition, it should be used to avoid confusion and the appearance of unprofessionalism. “Lightening” should never be used as a noun, period, but especially when what you really mean is “lightning.”
“lighten.” Merriam-Webster Online Dictionary. 2008. Merriam-Webster Online. 17 September 2008 <http://www.merriam-webster.com/dictionary/lighten>.
“lightning.” Merriam-Webster Online Dictionary. 2008. Merriam-Webster Online. 17 September 2008 <http://www.merriam-webster.com/dictionary/lightning>.
Wilson, Kenneth G. The Columbia Guide to Standard American English. New York: Columbia University Press, 1993. Bartleby.com. 17 September 2008 <http://www.bartleby.com/68/54/3654.html>.