There are two ways to approach that question.
First there is a legal definition, and there is commonly understood/philosophical definition.
Those two tend to overlap,
Legal definition would be ‘any being that is given human rights’, which doesn’t help at all as it’s not something set in stone.
Personhood is commonly understood to be the highest form of sentience, including such things as
ability to feel pain, self awareness, awareness of death, and unique to humans the ability to reason.
To my knowlage, this is also how most philosophers define it