In Digital two states known as '0' & '1'. Apart from these two we have another state that known as "Tristate".

i.e. In digital '0' for 0V to max of 0.4 to 0.8 V. '1' is taken account if it is minimum of sum 2.4V to max 5.0V. In between these 0.8V to 2.4V is known as "Tristate".