Within is a preposition.
Within means ‘inside or not further than a particular area or space’:
People who live within the city pay higher local taxes than people who live just outside the city. (= the people who live no further than the city boundary or limits)
We’ve always lived within ten miles of the coast. We love the sea. (We’ve always lived no further than ten miles from the coast.)
We can use within to refer to time:
I’ve booked train tickets on the Internet. They should arrive within three days. (no later than three days from now)
I’ve noticed her change within a very short time.
Within does not mean the same as in. Within stresses that something is not further than a particular area or space or not later than a particular time:
Your shirt is in the drawer.
Not: Your shirt is within the drawer.
I always have tea in the afternoon.
Not: I always have tea within the afternoon.
