Older mainstream rock music that is considered just wild and "rock-y" just for the purpose of being so. Dad rock is usually used in a derogatory manner towards rock music that (what are now) dads would listen to because it was popular at the time. Bands
like The Beatles or Black Sabbath or The Rolling Stones do not apply to this
definition because they are not trying to just rock it out; acts
like AC/DC or Guns N' Roses or Aerosmith are what usually gets tagged as dad rock instead because they feature typical stereotypes of older mainstream rock: wild vocal performances, focus on stand-out solos and a general sense of trashiness that comes from their sole purpose to be as obnoxiously wild as possible.
That being said, not all classic hard rock acts
get tagged as dad rock: even bands
like Led Zeppelin could apply to this
definition, but people don't usually call them dad rock because they're much more liked and respected than the other groups previously mentioned.
Dad rock is usually misused to describe literally any rock music that is older than 30.
"I don't listen to any of that
shitty dad rock; I only listen to good music
like my bloody valentine or The Velvet
Underground. "