A Progress Level (PL) is an indication of the state of technology that exists in a
particular society or civilization (which, in a science fiction setting, may be located on a planet other than Earth). This state of technological development generally pervades all aspects of a culture, particularly at higher levels (PL 5 and beyond) when long-range communication is virtually instantaneous. Even at lower levels, it's unlikely-but not impossible-for a group of humans (or other sentient beings) to be at one Progress Level in some respects and at another one in other respects.
Progress Level may vary wildly from place to place on the same world or even the same continent.