The Wikipedia article has multiple conflicting definitions, including:

"any use of the language, especially repeated phrases, to ward off forbidden thoughts”
"Claim Y sounds catchy. Therefore, claim Y is true."
"the start and finish of any ideological analysis"

The problem is that the term is just BS, in part because the idea it was made to support is complete BS.

Defining 'Totalitarianism' was a cold war project of western academia, trying to come up with a way to say that the nazis and soviets were the same. They weren't though. Only far right US Nationalists still claim this. The term has very low analytical use, so once the pressure to create this propaganda evaporated with the end of the USSR the term quickly became defunct.

Thought terminating cliches was coined by a psychologist in ’61 trying to claim that 'totalist thought is characterized by thought terminating cliches.' To translate: the west has reasoned ideology, everyone else just spouts cliches.