Pass a milestone or critical point, begin to recover. For example, Experts say the economy has turned the corner and is in the midst of an upturn, or The doctor believes he's turned the corner and is on the mend. This expression alludes to passing around the corner in a race, particularly the last corner. [First half of 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.