Basically, a group of creatures that all have one connected mind. There's not individual thought most of the time.

The Joining is a good example.

A group of people or creatures with a connected mind, often controlled by a puppeteer so to speak. Examples include the Geonosians from Star Wars. There's also different Symbiote Hiveminds in Marvel, such as Poison or Knull's Symbiote Hivemind. Even the Flayed from Stranger Things is an example of a Hivemind, with The Mind Flayer being the puppet master.