Is there a standard definition of a stopover, in flight rules?

Example:

A-B - daily or more
B-C twice weekly

If I book a 'no stopover' fare, and choose to book a flight from A to B that leaves me with 3 days to the next flight to C, is that a stopover?

Secondly, if you book an open jaw, which stopover rules do you follow, if they differ on the outbound and inbound?

E.g.,

"NO STOPOVERS PERMITTED ON THE PRICING UNIT." - outbound
"2 FREE STOPOVERS PERMITTED ON THE PRICING UNIT - 1 IN EACH DIRECTION IN B. " - inbound

Does that mean you can stop in B on the inbound?

How about
"1 FREE STOPOVER PERMITTED ON THE PRICING UNIT INBOUND" - outbound
"NO STOPOVERS PERMITTED ON THE PRICING UNIT" - inbound

And what basis do you use in the following case:

A-Z fares in the cheapest booking class are $500 return until 12th December, then $800 in the cheapest booking class from 13th December
B-Z fares are $500 return until 18th December, then also $800 for departures after 18th December.

You fly the open-jaw B-Z-A flying from B on the 13th of December, returning to A within the rules a couple of weeks later, in both cases booking into the cheapest fare class. Do you pay, for the return leg, 1/2 of $800, or 1/2 of $500?