All CMOS inputs must be terminated at either Vcc or Ground.
The inputs of a CMOS device are high-impedance. These terminations can be through a resistor (for example, a 10kohm pull-up resistor to Vcc or a 1kohm pull-down resistor to ground), or they can be directly connected.
CMOS outputs are typically best left floating.
CMOS I/O Pins
In transceivers (where inputs and outputs share a pin), the output can be terminated to ground or Vcc through a resistor (10k, 100k, etc).