The USB specification defines bus states, rather than logic levels on the D+ and D- lines.
For a full speed device, the bus state where the D+ line is high and the D- line is low is defined as the J state. The bus state where D+ is low and D- high is called the K state.
An idle bus, where D+ and D- lines are only polarized through the pull-up on D+ and pull-downs on the USB host side, will be in J state.
Both lines low are called SE0 (single-ended 0), and both lines high SE1 (single-ended 1).