Aha moment

a point in time, event, or experience when one has a sudden insight or realization:
My aha moment was when I put on my shorts from last summer and they didn’t fit.
Contemporary Examples

And in this aha moment, she explains in ways we all can understand what it must feel like to be transgendered.
Fenton Bailey and Randy Barbato’s Favorite World of Wonder Clips (VIDEO) Fenton Bailey, Randy Barbato February 4, 2013

Was there an “aha moment” when that balance clicked into place?
Grant Heslov Is the Robin to George Clooney’s Batman Andrew Romano February 6, 2014

“It was an aha moment for me and motivated the research,” Lee said.
The Gubernatorial Glass Ceiling—and What It Means for Hillary Clinton Eleanor Clift June 4, 2014

“aha moment” is quite old—our earliest citation is from a 1939 psychology text—but it was used only sporadically for a long time.
New Words Added to Merriam-Webster Dictionary: ‘Man Cave,’ ‘Sexting,’ and More Kory L. Stamper August 13, 2012

noun
an instant at which the solution to a problem becomes clear
noun

a sudden understanding, recognition, or resolution; also called [aha reaction]
Examples

Once she had that aha moment, she knew how to proceed.
Word Origin

1939