How to define a condo association or HOA board meeting?

What officially constitutes a board meeting? I have been told that any meeting among board members constitutes and official meeting? Does a casual conversation on the sidewalk or at the pool between a quorum of board members constitute an official board meeting? Could a decision made in that conversation be accepted and ratified at the next board meeting? Is an official agenda, posted in the proper time frame, the only thing that constitutes a board meeting?