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.
Look at this idiot trying to mansplain
periods to women... he thinks women only have
nine periods a year... wait 'til you hear his lecture on how to insert a
tampon correctly!