The Brief: TERF stands for Trans Exclusive Radical Feminist and refers to people who consider themselves to be feminist activists, but who do not include trans women in their activism.

DEEP DIVE
TERF stands for Trans Exclusive Radical Feminist and refers to people who consider themselves to be feminist activists, but who do not include trans women in their activism, or in their definition of women.
terf: Im a feminist. I believe women aren't defined by society and can act/dress how they want.
Trans Woman: *breathes*
terf: If YoU wANt Me tO tReat YoU lIkE a WOOOMMAAN you BetTtEr fIt thE ExAcT StErioTypIcal DeScripTiOn oF a WoMaN— Morte Divine (@MorteDivine) July 19, 2018
My favorite terf argument is "if 'trans women are women' then all women are trans women!", courtesy of someone who's still unclear on the relationship between rectangles and squares
— Lubekeeper Zinnia (@ZJemptv) July 18, 2018