Adventure generally refers to voluntary engagement in new things, especially things small or large, that might be somewhat daunting. Anything beyond emotional or physical comfort. Heart rate maybe.
I'll invite your views at the end of this, and my thoughts are far from fully-formed. Mrs. BD is much to the right end of a spectrum than I am - she seeks new things and challenges more than I do. It was her idea to hike in the Atlas mountains with Arab guys, not mine.
I think of adventurousness as existing on a spectrum with adrenaline junkies on one end and stick-in-the-muds on the other. I suppose courage is some aspect to it, and curiosity another. Life danger might be a different topic - or maybe not. Is combat, or facing a charging Cape Buffalo with a rifle - adventure? Is taking a new job adventure? Probably.
I am probably on the middle of the spectrum. For some personal examples, travel alone feels aventurous, travel with Mrs. or others does not. When I was young, asking a girl to do something felt adventurous even though these things usually worked out ok due to my charm (?). My first day in the gym with a trainer felt adventurous. Riding a horse or hiking on cliff edges feels crazy to me. Bouldering is pleasure, not adventure for me, so it's complex. Taking our boat longer distances feels like adventure.
A doctor once told me that the bravest person he ever knew was a severely agoraphobic woman who left her home to a supermarket.
I can think of many areas of adventurousness: creative, romantic, physical, intellectual, geographical, sexual, spiritual, etc. etc. Do "the kids these days" prefer virtual thrills? I dunno. Cheap thrills?
How do our readers view adventure? Let us know in comments.