When a
man explains something to a woman, uninvited, about a topic she is either equally qualified or more qualified to talk on.
A male
doctor explaining something to a female patient, for example, would not be mansplaining.
Examples: The
guy who tried to tell Margaret Atwood about "The Handmaid'
s Tale", the guy who told Katie Mack, who has a
PHD in Astrophysics, to "learn some science" because he thought climate change was a hoax, the
man who tried to correct a woman about how to pronounce her own name correctly, an Englishman correcting a Welsh woman on her Welsh pronunciation.
See also Dunning-Kruger effect.