Sidequel: a spinoff or sequel that exists in the same universe and chronological frame of time as its predecessor work.
This is easy with TV; spinoffs have been a common practice since nearly the invention of the medium. Concurrent main/spinoff TV series are less common, but there are many examples. Occurrences in film are rare; I could only come up with one (the first example below).
Examples:
The Bourne Legacy occurs concurrently with the events in The Bourne Ultimatum.
Agents of S.H.I.E.L.D. occurs concurrently with the Marvel films, a situation that is almost certainly unique.
I am pretty sure there was a direct to video Lion King movie that takes place chronologically within the original movie (a side adventure we never saw).
In relation to the movie “Soldier” with Kurt Russell was the first time I heard the term “sidequel.” The definition there was “same universe but not having to do with the original.”
“Soldier” takes place in the same universe as “Blade Runner”
My favorite is a sidequel from the movie The Incredibles. In which Elastigirl makes a call to her babysitter from her plane just before it’s hit by missiles. We get to see the phone call from the babysitter’s side of it.
Here is one from records. In the second Firesign Theatre album, Nick Danger receives a wrong number call trying to order pizza - pizza to go with no anchovies. The call is placed by a hungry George Tirebiter in the third album. Though it is a bit odd since Nick Danger runs on December 7, 1941 and the other is after Korea. But it might count.
I’ve got a TV example starring Harry Morgan before Dragnet - though it might be he is involved with 2, since Adam 12 is at the same time as Dragnet, while After-MASH is after MASH and so doesn’t count.
Disney is going to be generating a bunch of Star Wars “sidequels” beyond the Abrams Episode 7, 8, 9 films. Plus there’s the “Rebels” animated series. They want that money Marvel is getting for themselves!
I’ve seen it in books a few times. C. S. Lewis’ A Horse and his Boy takes place entirely within the time span of The Lion, the Witch, and the Wardrobe. And David Weber’s Honor Harrington series has spawned a few spin-off series that are better than the originals.
In the movie Gravity, Sandra Bullock’s character is trying to communicate with somebody, anybody, on Earth, and she reaches an Inuit speaker. Neither speaks the other’s language. Jonas Cuaron, Alfonso’s son, created the short film Aningaaq, from the Inuit guy’s point of view.
The Wind Done Gone by Alice Randall is a parody of Gone With The Wind. Randall’s novel parallels Mitchell’s book but does it from the viewpoint of a slave who is Scarlett’s half-sister.