Thresholds in Static discipline
\$ V_{OH} \$ is the minimum and \$ V_{OL} \$ is the maximum voltage of an output in the high and low states respectively when driving a specified load.

\$ V_{IH} \$ is the minimum input voltage that is guaranteed to be read as a '1' and \$ V_{IL} \$ is the maximum input voltage that is guaranteed to be read as a '0'.

Generally \$ V_{OH} &gt; V_{IH} \$ and \$ V_{OL} &lt; V_{IL} \$ for any given logic family to ensure reliable operation allowing for some voltage drop along the connections between devices.