Best definition
When you deflect criticism by pointing out flaws in your opponent, specifically using the phrase “what about x?” This is

an attempt to excuse you from changing you behavior by painting your opponent as a hypocrite.

Dad: Son, you haven’t cleaned your room in two years. Please clean it up.

Son: Why are you always telling me to clean my room? What about YOUR room? Or what about your relation with mom? Shouldn’t YOU be the one cleaning up your life?

Dad: Quit your whataboutism and clean your damn room. I’m tired of seeing that dead rat over there in the corner.

whataboutism: define #2
When you deflect criticism by pointing out deficiencies in your critics, specifically using the phrase “what about x?”
Dad: “Your room is messy. Please clean it up.”

You: “You’ve always been criticizing my room! What about YOUR room? Or what about mom? Shouldn’t YOU be the one cleaning up you life?”

Dad: “Just clean your damn room. Damn whataboutism.”
whataboutism: define #3
A method commonly used to dismiss arguments which demonstrate that your position is not based on principle, or is hypocritical in nature. Often used by social justice activists when faced with facts which do not fit their agenda or narrative.
“The rise in the suicide rates amongst women clearly shows the terrible effects of the patriarchy”.

“But 80% of suicides are male…”

“Oh here we go with the whataboutism. You’re clearly a misogynist.”
whataboutism: define #4
A rhetorical term used by Americans to dismiss criticism of their own hypocrisy. Originated when the Soviet Union would respond to American criticisms of Soviet human rights by pointing out, for example, that if America was so concerned with human rights, it should repeal its Jim Crow segregation and address its own white supremacist violence. Americans, believing their country was, by definition, “the home of the free” and above such criticism, accused Soviets of “whataboutism.”
American: “Cuba is a brutal, authoritarian dictatorship which suppresses political dissent.”

Cuban: “You know the one place in Cuba where political prisoners are detained indefinitely without charge? Fucking Guantanamo Bay.”

American: “Waaaah! That’s whataboutism! “
whataboutism: define #5
A rhetorical tactic to pivot from an accusation of hypocrisy. It is most commonly used to justify selective outrage in political discourse. The tactic is based on the Nirvana fallacy, the informal fallacy of comparing actual things with unrealistic, idealized alternatives.

D: Trump accepted $10 million from Russia.

R: Hillary accepted $200 million from Saudi Arabia and China

D: Whataboutism.

If the user of the word truly believed foreign donations was a political deal breaker, they wouldn’t have began with the criticism in the first place, as their preferred candidate is more guilty of the offense they are supposedly complaining about. They hold one politician to perfect standards, but another to lower standards, or none at all.

The “whatabouter” is not necessarily defending the perceived offense, but is mis-characterized as supporting it. As the only other choice in the election is a greater offender, the “whatabouter” is simply pointing out the implied criticism makes no sense when there are only two choices.

The tactic is a side effect of solipsism, the idea that only your own opinion exists or matters. As the user places no value on opposing world views, they are confused as why the shaming tactic behind “whataboutism” is ineffectual, and ascribe the ineffectiveness to stupidity.

“I can’t believe anyone voted for Trump.” “Yeah, they use whataboutisms to defend him”
whataboutism: define #6
Made up term created by US propagandists to defend their and their government’s hypocrisy.
US propandist: this is an evil man because he has tortured people!

Concerned citizen: what about US torturing people in Guantánamo Bay?

US propandist: Whataboutism!