Friends and Mad About You had a crossover.
I don't personally see it as a crossover. More of an establishment of continuity between the shows, stemming from the fact that Lisa Kudrow played a recurring loopy waiter on Mad About You and a loopy masseuse on Friends. But it was mostly one way, with Ursula making multiple appearances on Friends and Helen Hunt's character also making a cameo appearance where she, of course, mistook Phoebe for Ursula.
But there was a sort-of crossover of the Thursday shows, where a blackout was a central theme to all the shows. I remember that as Chandler was trapped in an ATM vestibule with Jill Goodacre.
So anyway, I look at "crossovers" as sort of a multi-level thing. There's the establishment of a shared universe via a quick cameo or guest appearance or a character with a connection to another character. Examples would be the Phoebe/Ursula connection, Kensi Blye showing up for 2 minutes on Hawaii Five-O before the NCIS: LA crossover, or Digg showing up on The Flash. I think the entire St. Elsewhere thing is based on this.
Then there is the meatier crossover, where multiple characters from a different, established show show up on another show, possibly stretching into both series. The Arrowverse crossovers are obviously a modern standard for this, but you had stuff like Law and Order/Homicide.
I think backdoor pilots are a bit of a different animal, as they're clearly created to introduce the spinoff series to the audience. And sometimes those series aren't even ordered.
And let's not forget Star Trek's version of a "crossover", which basically means going to the Mirror Universe.
