Unintentionally happening at the same time or in the same way as something else. By coincidence, my sister and I bought our mom the same sweater for her birthday. I didn't just come to the same place as you by coincidence—your assistant told me where I could find you.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
by an accidental and strange similarity; by an unplanned pairing of events or occurrences. We just happened to be in the same place at the same time by coincidence. By coincidence, the circus was in town when I was there. I'm glad because I love circuses.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
Through an accidental simultaneous occurrence, as in By coincidence both researchers discovered the same solution. [Mid-1600s] Also see by chance.
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.