Simultaneous Voice and Data Only
A protocol and technical standard that allows a CDMA phone to use both voice and data networks at the same time. Specifically, when the voice network is CDMA 1xRTT and the data network is CDMA 1xEV-DO, commonly referred to as CDMA 3G.
SVDO is important because CDMA EV-DO 3G (and LTE 4G) networks were initially only built to handle data, not voice. Without SVDO, data service is unavailable during a voice call.
Not all CDMA phones support SVDO.
SVDO only applies to 3G (EV-DO) networks. A different but related standard - SVLTE - applies to 4G LTE data networks.