In the play A Doll's House, what is the difference between Torvald's and Nora's definition of a "human being"?
Torvald does treat Nora as a doll. Ibsen's A Doll's House has often been used as a rallying cry for the feminist movement, especially during the second wave when Betty Friedan's Feminine Mystique was a pivotal point in that part of the movement. While Ibsen has considered himself more of a humanist instead of a feminist, there's no denying that he wanted Nora to be an equal to her husband.
In the final act she says, "But our home's been nothing but a playpen. I've been your doll-wife here, just as at home I was Papa's doll-child. And in turn the children have been my dolls. I thought it fun when you played with me, just as they thought it fun when I played with them. That's been our marriage, Torvald. [. . .]If I'm ever to reach any understanding of myself and the things around me, I must learn to stand alone. That's why I can't stay here with you any longer."
Torvald does not consider his wife his equal, he considers her more like a doll or a child. Something to play with, feed macaroons, have dance for him, and call his "little squirrel" and other diminutive names. Most likely, the only others that Torvald considers equal to him are other men of his status as a businessman (or greater) in society.
Nora, as you can probably tell, defines being human as someone who understands herself and someone who is free. It's not about status or gender to her.