Abstractly, a client in the Domain Name System (DNS). More concretely, a resolver is software running on a TCP/IP host that enables the host to query a name server in order to resolve a host name into its associated IP address.
The resolver software enables the host to formulate and send a query to the name server, interpret the response from the name server, and pass this information to the program that initially called the resolver software.
On UNIX platforms with BIND implemented, a resolver is a set of library routines that are linked to the client programs that need to use them, such as Web browsers or File Transfer Protocol (FTP) client programs. Computers running Microsoft Windows become resolvers when you install the TCP/IP protocol stack and configure the IP address of a DNS server on the DNS tab of the TCP/IP property sheet.