A highly developed civilization that developed in what's now Pakistan is referred to as the what?
I believe you are referring to the Indus Valley Civilization, also known as Moha-jo-Daro and Harappa civilization, so named after the first two sites at which remains of these civilization were excavated. The exact history of origin and development, and decline of these civilization is not very clear. Earlier these civilizations were supposed be the part of Aryan civilization who were supposed to have migrated to India displacing the supremacy of the Dravidian people who were supposed to have been original resident of India, and who were forced to migrate to south India because of Aryan invasion. However now other alternate theories have also been developed.
Most probably the Indus valley civilization is a part of a phase of civilization that might have originated about fifteen thousand years back in eastern Afghanistan at Mundigak and western Pakistan at Mehrgarh, Starting from there a string of civilization sites have been found to indicate that people from this civilization successively abandoned their older locations and moved toward east and south over a period extending more than ten thousand years. In this way they reached many places in in central India beyond Delhi (Alamgirpur) and North Maharashtra India upto Daimabad. With passage of time and place changes took pace in their ways, but the signs that these represent gradual evolution of the same original civilization are very clear.
A good comprehensive description of this civilization is given at the web site referred below.