The definition of part time usually comes down to credit load.
Look at it this way - if a full time degree program takes x number of years, and is worth y "credits" (some universities actually use this terminology but it doesn't make a difference if they don't) then the number of credits per year representing a full time program is y/x. A full time student should therefore be studying for y/x credits in any year to be considered full time. If there are compulsory internships or if credits are available for work experience then these contribute to the credit load. If a student arbitrarily takes a break for any reason, including self-imposed work experience then that would not contribute to the credit load.
For example, at University X students have to take 120 credits a year for 3 years - total credits amounting to 360. If they were part time they might have been doing 60 credits per year for 6 years.
At a university or school that doesn't operate a credit or module system we can say that the whole degree equates to 1 credit. If the program typically takes 3 years to complete on a full time basis, the credit load should be 1/3. Part time students will have a lower credit load as they are taking longer to complete the program - perhaps it would be 1/5 or 1/6.
Perhaps a simpler way to look at is in terms of velocity. If a given student works at their current commitment will they qualify in the typical time period allotted to a full time program (assuming no fails or retakes)? If so, they are a full time student, if not they are part time.