Well, first, a clarification. I said that in my campaign world, people were not considered adventurers. I implied that adventurer as a profession really had no place in the culture of my campaign world. I did not imply that adventurer as a profession did not exist in some historical cultures.
Notably, if your campaign is based off the British Imperial period, then you'd be quite in the right to have whole classes of gentlemen, bored gentry, and bandits and plunders pretending to be gentlemen out adventuring - which could mean anything from archeaology, to taking a wild vacation, to playing the game of thrones in some far corner of the world.
I said I had incorporated into my campaign certain medieval elements for the purpose of causing culture shock. Adventurer is a much more modern idea, and our notion of it more modern still.
Note that the 1474 definition is much the same as criminal, and I dare say that the best synonym would be rake. When players say that they are adventurers do they generally mean scoundrel?
The 1548 definition is basically 'merchant'. When players say that they are adventurers do they generally mean travelling merchant?
And the 1667 definition is basically 'mercenary'. This is better, but even so most D&D adventurers don't mean people - especially people of high birth - who volunteered to fight in a war for a country other than thier own. All such definitions are a good bit more recent than I want, and if we started looking into 19th century definitions they'd probably match right up with our D&D sensibilities.
Unless I'm deliberately designing a setting with a Victorian feel, I'm going to do my best to avoid 'adventurer' as a phrase, do my best to avoid the modernity of adventures as something people seek out and not unwanted things that happen to them, and do my level best not to have every barkeep a retired adventurer.
Concensus D&D inspired fantasy seems to have every third bloke an adventurer or a former adventurer, and the phrase got really tired and worn out for me very early in my gaming career. D&D 'adventurers' seem sometimes to be a class of itinerate sanitation specialists or big game hunters and ultimately the term seems to deprive the adventure of its adventurousness, and reduce the occupation of hero to the mundane level of rat catcher or safarii guide. Every second person seems to have a list of chores for an adventurer to perform and is not in the slightest unwilling to pay a stranger to do them. Realistic or not for a world with real monsters, it deprives the whole thing of its mythological power, especially if the profession is common enough to be considered a profession and if the PC's find that they have peers on every street corner. I sometimes wonder whether or not in such a society adventurers should have formal resumes, agents, and talent scouts looking for the next set of adventurers with the right stuff.
"That nice, but we were really looking for someone who could cast fireballs... NEXT!"
And conveinent as a plot device or not, it perhaps becomes to conveinent and too easy to keep hooking the PC's with 'a merchant lord wishes you to investigate the...in return for the generous reward of...'
Not that I haven't fallen into that trap myself, but I strive to keep the motivation more personal than that.