"First tooth" refers to the very first tooth that a baby grows in their mouth. It is a significant milestone in a child's development when they start getting their first teeth. Full definition
Good dental care begins before a baby's first tooth appears. (kidshealth.org)
It's important to take your child to a pediatric dentist and keep up with brushing his or her teeth as soon as that very first tooth comes in. (abckidsinc.com)