In the process of developing or growing, being made, as in The editor believed this election signified history in the making. This term is frequently used to describe the course of events, as in the example. [Mid-1600s]

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.