(noun): A preliminary discussion; a preface or foreword. Plural is "prolegomena". The adjective is "prolegomenous."
This is a somber [gloomy, serious, grave] term for sedate [dignified, serenely deliberate] occasions, "As a prolegomenon to this meeting, I would like for us to discuss the advantages of simply taking whatever they offer and levanting [run off without paying; “cair fora”]."
It sounds a bit misplaced in normal social intercourse, "Arnold proposed to me last night. As a prolegomenon to his proposal, he gave me an in-depth analysis of his financial situation."