There's no definition of "introvert" that psychologists agree on.
Non-psychologists may have their own definitions of "introvert", too.
It is not the same as being shy.
In fact, there are different kinds of introversion.
Social introverts genuinely prefer to hang out on their own or in smaller groups.
Thinking introversion applies to people who are very self-reflective.
Anxious introverts start to feel frazzled when they spend too long hanging out with other people.
Restrained introverts take a little while to get going.
People can become less introverted as they get older.
If you want to, you can train yourself to act more outgoing when you need to.
Introverts are not less happy than extraverts, despite what you might have read.