Oh no... sorry, Barry. Wasn't directed at anyone in particular.

Shifting gears,

If someone asked me to define it so that a layman could understand it, I'd tell them the "O" is an abbreviation for "Order of Magnitude" and the the formula it contains is nothing more than the part of another formula that's going to have the biggest effect on things... that's it's just shorthand to make comparing some more complex formulas easier to compare as a swag (order of magnitude).

For example, the forumula for the number of rows that a triangular join will touch in SQL Server is ((N²+N)/2)+N. If you strip away all the small stuff that won't impact the result much when the numbers get really big, you end up with O(N²/2). That's much easier to compare to a cartesian join which is both N² and O(n²) to say that a triangular join is approximately half as bad as the square join.