(1) A cellphone that operates on any cellular carrier's network that supports the same air interface (GSM or CDMA) and frequency bands built into the device. See
SIM card,
SIM lock,
jailbreaking and
Android rooting.
(2) A cellphone that is not password protected.
(3) A cellphone that was recently opened with the correct password and is available to anyone until it is locked manually or automatically.