To make or become bright or brighter.(verb-transitive)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "brighten" in a sentence
"Cretak allowed her expression to brighten, as if with great relief."
"Proponents urge us to "brighten" clouds by spraying up droplets of sea water, as scientists are now trying to do with a little help from Bill Gates."
"I like lime wedge or even a cucumber slice in my water ... do you have any favorite ways to 'brighten' a plain glass of water?"