SSID - service set identifier
SSID is short for service set identifier.
SSID is a case sensitive, 32 alphanumeric character unique identifier attached to the header of packets sent over a wireless local-area network (WLAN) that acts as a password when a mobile device tries to connect to the basic service set (BSS) -- a component of the IEEE 802.11 WLAN architecture.
The SSID differentiates one WLAN from another, so all access points and all devices attempting to connect to a specific WLAN must use the same SSID to enable effective roaming. As part of the association process, a wireless client must have the same SSID as the one put in the access point or it will not be permitted to join the BSS.
SSID is considered to be a fairly weak form of security because an SSID can be sniffed in plain text from a packet and most access points broadcast the SSID. Some network administrators turn off SSID broadcasting, but a hacker can still sniff the SSID from frames that stations use when associating with an access point.
An SSID is also referred to as a network name because essentially it is a name that identifies a wireless network.
Learn more about SSID and minimizing WLAN security threats on Wi-Fi Planet.