Please merge the following tags:
dhcpd is actually already a synonym of isc-dhcpd (whether or not this is ideal is something we can debate - I think it makes more sense to point it at dhcp-server myself, but merging isc-dhcpd with dhcp-server may not be the most correct thing to do).
The second synonym idea (isc-dhcp -> isc-dhcpd) already exists (someone beat me to it or it was already there?).
Honestly it should probably go the other way around -- the ISC DHCP reference implementation is both a client and a server, and someone could be asking a question about either component...