Location in Midgar
Sector 8 is one of the eight sectors of Midgar and one of the six sectors that you are able to visit. Check out the full Midgar location page for more information on the different sectors.
While Sector 8 plays a much more prominent role as a location in some of the other games in the Final Fantasy VII universe (such as Dirge of Cerberus, Crisis Core and Before Crises), it plays mainly a transitory role in Final Fantasy VII.
Sector 8 is where Cloud first meets Aeris at the beginning of the game. It is also where the team parachutes in to when they arrive on Disc 2 for the second visit back to Midgar. Check out the Midgar walkthrough page for more information.
You cannot revisit Sector 8 once you have passed these two sections of the game - meaning that if you left any important items behind ( Missing Score) there is no way to return.
The map below shows the location for Midgar on the World Map: