Jazz and pop music were practically synonymous in the US between the mid '30s and mid '40s. Later they diverged and evolved in different directions. But many of the standards of the Great American Songbook originated in the aforementioned period, the swing era, so jazz is inextricably linked with this type of music we’re discussing.
There’s certainly a blending of the traditions going on, but I feel that the music listed above as “traditional pop” doesn’t have quite the same improvisatory nature as jazz music (which is a good portion of what makes jazz what it is), and arrangements are perhaps more “set” than in jazz. It’s like early rock and roll definitely coming from the roots of blues (and country/western), but putting a different twist on it.
edit: Jazz of the 40s and 50s I would classify as music encompassed by the labels of bebop, hard bop, cool jazz, modal jazz, etc., as exemplified by Thelonious Monk, Charlie Parker, Miles Davis, Bill Evans, etc. Big band jazz is the sort of bridge between the small combo jazz bands and the traditional pop, and there it becomes a little more hazy in terms of defining genres.
I’m just looking at what’s going on in the '50s – I don’t think anyone, including Perry Como himself, would call Perry Como a jazz singer.
“Jazz” being such an ill-defined, nebulous term, while I agree that I would hesitate to call Como a jazz singer, it wouldn’t surprise me in the least if he’d considered himself one. The first time I was made aware of the distinction, I was interviewing a singer whom I’d always considered “lounge.” I.e., she sang standards and pop songs, but arranged with a tongue-in-cheek kind of bubble-machine, champagne swing. She sternly corrected me that she sang jazz, not lounge. And since that time, I’ve noticed the same distinction being made–sometimes defensively–by other singers of such music.
The best definition would be “Tin Pan Alley.” Most American popular music, came from Broadway and the movies. Before that it was popularized in vaudeville. The songs were written by people who usually did not perform them. “Tin Pan Alley” was the term used to describe the area of New York City, where the offices of music publishers were located.
It was common for songwriters to release new songs to lots of different artists then you’d have a race of “cover versions” to see which singer could have the most popular version of the song.
Also radio shows almost all had their own singers. Some like “Burns & Allen,” varied their singers from season to season, others like “Jack Benny” had singers stay on and become part of the show, like Dennis Day. Bandleaders like Phil Harris (Jack Benny) and Arte Shaw (Burns & Allen), provided instrumental hits for comdey shows.
Tin Pan Alley is hardly ever used as the name of a genre:
It’s a location and by extension the music publishers there and the songwriters there.