Cross-axis sensitivity or "angle orthogonal to the plane of measurement":
Cross axis sensitivity is a proportionality constant that relates the change of the inclinometer output induced by a cross acceleration/inclination input. The sensitivity can vary depending on the direction of the cross input.
For example: A liquid capacitive gravity based sensor needs to be mounted in a vertical position - if the back of the inclinometer is mounted 45° off of vertical one would expect the inclinometer to be accurate within 0.1° of the expected output when mounted vertical. Due to the unique cavity design the error is predictable without much overall change to the accuracy. The unit will still provide output along the sensor's degree range, however the error at larger cross angle inputs will be significantly higher - as much as 2 or 3 degrees - the further off vertical the sensor is mounted.
It is recommended to mount a gravity based inclinometer sensor as close to vertical as possible to maintain the integrity of the sensor's output.