To take by choice into relationship (a child, heir, friend, citizen, etc.)
"After years of waiting for a family to form, they finally adopted a young girl from the orphanage."
To take voluntarily (a child of other parents) to be in the place of, or as, one's own child.
"After years of fostering, Sarah and Mark decided to legally adopt their favorite foster son so he could officially become part of their family forever."
In plain English: To adopt means to legally take another person's child as your own so they become part of your family.
"They decided to adopt a rescue dog from the shelter."
Usage: Use "adopt" to describe the voluntary legal act of taking another person into your family or assuming a new identity for yourself. Do not confuse this with "adapt," which means to adjust to new conditions, nor with "abduct," which implies kidnapping.
The word comes from the Latin adoptare, which literally means "to choose for oneself." It entered English through Middle French with this same sense of selecting someone to be a child or heir.