The end of a difficult situation or task, the solution to a difficult problem. For example, It's taken three years to effect this merger, but we're finally seeing the light at the end of the tunnel. This metaphoric expression dates from the 1800s, but became widespread only in the mid-1900s.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Company. All rights reserved.