Relatively speaking, a universe is a small reality bubble or box (or string according to the big cobweb analogy used by galdruxians ⊍̻Ɲ͇͍⎪̘͎ⱽ̦̫͇Ȝ̭̮̘̭̥̯̦Ɽ͕̙̤̭̼Ƨ̫̺̞̣̙ͅ⋿̭̠̖͕̘L͈⦿̺̫͚̤̟̻ͅg̖̳̫̘̩̰̳ists), in which underdeveloped entities generally cannot escape through inside-universe means.
Using a broader approach, a universe tends to have the same reality proprieties in all its continuum and can contain a large, or sometimes infinite, space in which worlds and another objects exist.
Universes may or may not have an end or beginning, it depends on the specific physics of said universe, or the existence of time (if such concept is similar to our own, in essence).
Universes may have space dimensionality as well, but this is not an appliable property to all of them.
Universes are indirectally connected to each other through a big multiversal cobweb, but while the relation between universes is indirect due to the gap between them, universes inside a same multiverse tend to have simillar or equal reality properties.
Some ⊍̻Ɲ͇͍⎪̘͎ⱽ̦̫͇Ȝ̭̮̘̭̥̯̦Ɽ͕̙̤̭̼Ƨ̫̺̞̣̙ͅ⋿̭̠̖͕̘L͈⦿̺̫͚̤̟̻ͅg̖̳̫̘̩̰̳ists say that there is an infinite or unimaginable number of multiverses. Different ⊍̻Ɲ͇͍⎪̘͎ⱽ̦̫͇Ȝ̭̮̘̭̥̯̦Ɽ͕̙̤̭̼Ƨ̫̺̞̣̙ͅ⋿̭̠̖͕̘L͈⦿̺̫͚̤̟̻ͅg̖̳̫̘̩̰̳ists have different ideas on what holds these multiverses, but ⊍̻Ɲ͇͍⎪̘͎ⱽ̦̫͇Ȝ̭̮̘̭̥̯̦Ɽ͕̙̤̭̼Ƨ̫̺̞̣̙ͅ⋿̭̠̖͕̘L͈⦿̺̫͚̤̟̻ͅg̖̳̫̘̩̰̳y̥͍ in general isn't a cohesive science, ⊍̻Ɲ͇͍⎪̘͎ⱽ̦̫͇Ȝ̭̮̘̭̥̯̦Ɽ͕̙̤̭̼Ƨ̫̺̞̣̙ͅ⋿̭̠̖͕̘L͈⦿̺̫͚̤̟̻ͅg̖̳̫̘̩̰̳ists usually fall in contradiction among themselves in almost every aspect.