CGRAM stands for Character Generator RAM.
Character LCDs include a fixed CGROM to define the bulk of the characters they display. However, since the CGROM cannot be changed the designers also included the CGRAM — a small number of characters (typically 8) that can be redefined at run-time.
These eight characters are usually mapped to characters 0DEC (0x00HEX) to 7DEC (0x07HEX). The CGRAM definitions can be used to make small animations, bar graphs, and similar small graphic or sprite images.
This term very likely has its beginning in the granddaddy of all LCD controllers, the venerable Hitachi HD44780.