A leased line is a dedicated, symmetric fixed-bandwidth connection between two sites.
Businesses use leased lines to connect their offices to the Internet, and to connect offices to other offices.
A leased line is a dedicated connection. That means that the bandwidth is reserved solely for use by one company. The connection won't slow down at peak times.
Leased lines are symmetric. They can send data just as quickly as they can receive it. That contrasts with most ADSL connections which offer a fast download speed but a very slow upload speed.