When a man explains something to a woman that she already knows.

Usually in a manner in which it is clear that he thinks he is much more knowledgeable than her on the subject and that she has childish intelligence.

For example, a doctor meets a man in a bar and he asks her what she does. She says 'a doctor' and he replies; 'Ah! The thing about health and the human body is...' and proceeds to explain what it's like to be a doctor to her. Even though she is a DOCTOR and he is not.

When a stupid person (in this case a female) asks a distrubingly poorly thought-out question and is then blames her embarrasment, using the “mansplaining” scapegoat, on the fact that a somebody (in this case a male) answered their question sufficiently.

Basically, It’s every time a man talks to a women. (Experienced it first hand)

1st of all It's not a thing, for all sane human being.
Mansplaining is a word invented by Modern Feminist that they try to put the world Explaining in to the sexual term to get advantage from other aspects and get support from Modern feminist or other females who fallen in to this trap to fight an argument against male when they are losing the argument or just don't like whatever the male is explaining or getting in triggered stage and don't know what to do to stop the male from Explaining things they don't like or don't want to hear.

OR
Tl;dr : Its just a version of "explaining" with sexual in it to make males look bad and to draw support from other females.

A term some retarded people at buzzfeed made up. Used for when a man explains something, and they don't like the way he used a sentence structure, so they claim he is being condescending.

A feminist's only comeback

Basically, just plain explaining. It is a term created by radical females who refuse to acknowledge and give credence to any logical male with convincing rhetoric because of said female's penis envy and therefore, to spite the opposite gender who she feels is given everything on a silver platter, is in a state of denial and refuses to acknowledge any success brought on by their natural gender or gender-neutral talents. She perishes any thoughts that men also have problems of their own and can envy what women are capable of. Her insecurities shaped a psyche that just cannot allow her to accept the success of any male; she is just plain jealous and doesn't wish to concede due to her own poor life choices rather than use her talents to her abilities.