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.
[reads post] Yep, I'm using it right.
What do all serious auths have in common? The plan to use mass murder, at some point or another, as a means to an end. Unironically, my favorite thing about Tankies is how they'll go from "communists aren't violent murderers actually" to "you'll get the bullet, too" in the span of two posts.
To clarify my position, I'm more closely aligned with anarcho-communists than any other serious political movement. I just think authoritarians of all stripes suck.