Bellatrix doesn't know what she is.
The obvious title would be Seer, but Bellatrix knows that's not it. Seers does not have visions of absolute future, they have glimpses of future that can be or can not be, or they make prophecies and that's it.
But what she sees is not predictions, she does not read tea leaves or look at crystal balls, no it just comes to her and they are always, always true. Everything she sees, to the last detail always happens. And Bellatrix does not understand what that means.
The first time she sees something she is eight, and it is something so trivial that she just thinks it was a dream. The second time and third time is the same. But the fourth time she is with her mother, and when the vision finishes, she opens her eyes to her mother shaking her, shouting and shrieking and there is almost tears in her mother's eyes and she is just so confused. Later, when she talks with her mother about it happening before, her mother tells her that her eyes had turned completely white and she was shaking and muttering.
Her mother tries her best to find out what it is too, they even tell her grandmother to look at the Black library but they just never find what she truly is.
When she is eleven, just less than a year before she starts Hogwarts, Bellatrix has another vision. She is a mess by the end of it, and she just runs to her mother to describe it and the two leave their house the moment Bellatrix finishes her explaining.
Bellatrix is the one finds her.
She walks in to the drawing room to find her grandmother lying on the ground with lifeless, open eyes and a smile on her mouth and her wand is on the ground and Bellatrix just screams and screams and scream-
She learnes that day that seeing the future, knowing the future is not something she wants.