What is a general definition for pseudo-measurement (e.g., zero-velocity update as often used in robotics)?

I understand what a pseudo-measurement, but I am curious if there is a formal definition of the pseudo-measurement. A pseudo-measurement is sort of a constraint on the states of a system, which can be applied through a measurement. However, this is rather informal and unclear.

A simple example is a zero velocity or zero angular rate pseudo measurements. For example, if a robot is stationary, the velocity and angular rate are zero, so an update can be applied to the states (e.g., using a Kalman filter).