The definition varies between communities/fandoms, but generally a demi-human is considered to be a human sub-species with foreign physical features, heightened senses, special abilities, nonstandard colors and/or a nonstandard lifespan. They are mostly human in appearance and exhibit an intelligence similar or equal to humans.
Demi-human is a general term that includes fantastical species such as elves, giants, angels, dwarfs, merfolk, and even animal-related variants like wolf-, fox- or cat-folk (the list goes on).
There are countless shows, games and anime featuring demi-humans, a few examples would be Lord of the Rings (show), The Elder Scrolls (video game) and Goblin Slayer (anime).
Demi-humans are not equal to anthropomorphic animals despite some having animal features, and most furries agree that demi-humans are not part of their fandom.
Example:
Human traveler: This is the most diverse town I have seen so far... I have never seen more demi-humans in one place. And I see that you finally brought your dwarf friend with you.
Dwarf: He is a halfling, our species get confused a lot, but yes you can refer to us both as demi-humans.