A guardian is someone who looks after and protects another person or thing.
When we talk about guardians, often what comes to mind are children. For example, if a mother can't take care of her baby anymore, she might ask her sister to be the guardian until the child grows up. In this case, being a guardian means making sure someone's basic needs are met, like food and shelter. But a guardian can also be responsible for someone's well-being in other ways – maybe an older person is their power of attorney and makes decisions about their health care or financial matters. Even animals can have guardians; sometimes zoos will take on the role of caring for injured wildlife until they're healthy enough to return to the wild.
Someone who guards, watches over, or protects.
