"Pop" used to mean anything non-Classical, but these days I think of it in terms of short(er) songs with good melodies, hooky sing-along choruses, and the classic verse/chorus/verse/chorus/bridge/verse/chorus structure. Pop was "the" style of music teenagers were listening to in the late-50's/early-60's, but it was in 7" 45 RPM singles format. The British Invasion changed that overnight.
My long-time favorite Pop albums include early Beatles, Emitt Rhodes’ debut, Sincerely by The Dwight Twilley Band, all the Rubinoos albums, Squeeze, The Records, etc.
The mention by @slaw of Moon Martin brought a smile to my face. He was a great songwriter, his albums being an excellent blend of Pop with a hint of 1950’s Rock ’n’ Roll. In 1979 my then-wife started running Moon’s fan club, so I saw him live many times. Once was opening for Rockpile at The Country Club in Reseda, California, a great show.
Another band combining Pop with 50’s R & R are The Flamin’ Groovies, a semi-garage band I love. And I consider The Ramones a Pop group.