This is relevant to Question 24from the first paper of 2005: "For each of the following terms related to pressure monitoring, provide a definition and outline their role: zeroing, levelling, and calibration."
"Zeroing"can also be defined as "the use of atmospheric pressure as a reference standard against which all other pressures are measured".
The canonical college definitions is "a process which confirms that atmospheric pressure results in a zero reading by the measurement system"
"Levelling"can be defined as "the selection of a position of interest at which the reference standard (zero ) is set".
The canonical college definition is "a process which determines the position on the patient you wish to be considered to be your zero."
"Calibration"can be defined as "an adjustment of system gain to ensure the proper response to a known reference value".
The canonical college answer is "a process of adjusting the output of a device to match a known input value"
An excellent online anaesthesia textbook offers some definitions.
LITFL also have a good page on this material.