1. The stage after adolescence
2. Legally, someone who is (usually, depending in what country you live) 18+.
Now, there is a difference between legally and biologically.
At 18, in most country, you are legally an adult.
But, since most people continue to grow in some way at 18, they can't be considered adults physically.
Even if most people at 18 are "physically" teenagers(they continue to grow), you're still considered an adult, but it's not rare that people who are 18-19 are called "teens" (probably because of the number, eighTEEN
It is confusing, since at 18-19, you're both an adult(legal) and teenager(physically).
Because of that confusion, people of that age (18-19) are generally called "young adult".