Tankie's original use was for British communists who supported Soviet military expansion. In the modern sense, it is used to describe communists who are authoritarian-apologists. For example, a communist who romanticizes the Soviet Union or makes excuses for the Uyghur genocide is a tankie. I've also seen it stretched to include militant anti-capitalists, or more commonly, "militant," anti-capitalists who call for violent resistance to capitalism from the safety of a keyboard.
Democratic-Socialists are not tankies. Socialists are not tankies. I don't even think most communists qualify as tankies. Criticizing Democrats does not make you a tankie. Condemning Israel's human rights violations does not make you a tankie. Voting third party doesn't make you a tankie. I see this term used here every day, but never correctly.
Tankie: a Leftist that may self-identify as a Communist, or erroneously, a Socialist. Typified by their usage of a Hammer and Sickle or Joseph Stalin as their profile picture, these poor souls believe that the only way to achieve a Proletarian Revolution is to centralize power and control the flow of information similar to Stalin's Russia or modern day China. This includes the threat or actual use of violence on their own citizens in order to maintain their version of the status quo. The ownership of the means of production will be nationalized. 3rd party government oversight would not have even been thought of. Basically fascists but with a red coat of paint.
"erroneously a socialist" oh come on. All communists are socialist but not all socialist are communist. You can't just change definitions of words because you are a left-anticommunist. I don't even know what to say besides read a book.
Did you read the self identify part?