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.