Download acceleration, also known as multipart download, is a term for the method employed by software such as download managers to download a single file by splitting it in segments and using several simultaneous connections to download these segments from a single server.
One reason for doing so is to circumvent server side limitations of bandwidth per connection. Another is fault tolerance: in case of a failed connection only a part of the file transfer has to be redone. Because in normal networking situations all individual connections are treated equally, rather than actual file transfers, multiple connections yields an advantage on saturated links over simple connections, both in terms of total bandwidth allocation and resilience. Many servers, however, implement a maximum number of simultaneous connections per client in order to mitigate this.