The term "TERF" has been used so much in the past few years, and it has been used as an acronym, to the point that the original meaning has been lost. Because a TERF is a "Trans Exclusionary Radical Feminist", but most people described as TERFs are not, in fact, Radical Feminists.
I am not exactly an expert on the matter, but if I had to describe Radical Feminism (and Radical Feminists), the simplest description is that radical feminism is the belief that modern institutions, including things like industry, education, medicine and goverment were created by and for the male power structure and that women can only enjoy freedom and welfare under a radically different way of life. This is in contrast with Liberal Feminism, which believes that women can enjoy freedom and welfare by gaining access to those power structures, although in some cases, some of those power structures need to be greatly reformed. For a liberal feminist, a woman being appointed CEO of a corporation is a victory. For a radical feminist, this woman has been corrupted by a male dominated power structure, because corporations are intrinsically hostile to how women are, as well as to the earth.
Radical feminism is also essentialist, in that it believes there is an essence of being a woman that is irreducible and unchangable. (Although they usually wouldn't put this in terms of chromosomes because those are the province of reductionistic male science).
To paraphrase John Goodman in The Big Lebowski: "Say what you want about the tenets of radical feminism, but at least it is an ethos". Radical feminists believe that maleness is an irreducible essence, and that it also corrupts everything with hierarchy and violence. The original trans exclusionary radical feminists believed in excluding transgender people because they were unalterably corrupted by the violent essence of masculinity. I don't agree with this, but at least it is an ethos.
So the problem is that the term has been picked up and used as a general description of transphobes. A quick google search, for example, has many examples of people describing J.K. Rowling as a TERF, for example. But Rowling is not a TERF, because she is not a radical feminist. She is someone who has made billions of dollars through deals with large media companies, and whose writing and other expressions show that she generally supports modern institutions, even if she wishes to reform them. Most people who are described as TERFs are the same: either liberal feminists, or not feminists at all. Part of this is because there really aren't that many radical feminists out there. A soccer mom in South Carolina who is worried about transgender students playing high school sports is in no way a TERF, and is probably not even a feminist at all.
Words change meaning, and the word "TERF" will probably continue to drift from its original meaning, but I did want to note that it did originally mean something specific.
And also I realize that many of the simplifications I made here will not sit well with everyone.