Extended Service Set ID
The ESSID is the identifying name of an 802.11b wireless network. By specifying the ESSID in your client setup is how you make sure that you connect to your wireless network instead of your neighbors network by mistake. 
The Extended Service Set Identification (ESSID) is one of two types of Service Set Identification (SSID). An Ad-hoc wireless network with no access points uses the Basic Service Set Identification (BSSID). In an infrastructure wireless network that includes an access point, the Extended Service Set Identification (ESSID) is used - although it may still be referred in a loose sense as SSID. Some vendors refer to the SSID as the "network name".
See also: AccessControl