Quirkless is a term that describes someone who was born without any Quirks whatsoever in the My Hero Academia Universe.
Quirkless people exist as a minority in the My Hero Academia universe, possessing absolutely no Quirk, particularly unique features, or anything that would put them under the classification of being superhuman. They appear to be the subject of mockery and a level of prejudice as shown by Izuku's treatment in the first few chapters of the story. Commonly, anyone who does not possess a Quirk is considered to be weak and worthless, or that their circumstances are incredibly unfortunate.
As seen in the beginning of the series, people who are born with Quirks do not have an additional joint in their small toe, which is what evolution trades in exchange for their Quirk. This does not appear in those who have no Quirk, which is why this occurs. Quirkless people make up approximately 20% of the world, and appears to be a recessive trait, meaning that more often than not, if two parents, one Quirkless and one possessing a Quirk have a child, that child is far more likely to be born with a Quirk. However, as was seen with Izuku in Chapter 1, this does not appear to be a fast and true rule, as his parents both had Quirks, but he inherited neither.
It is possible for a Quirkless person to inherit a Quirk, such as One For All. However, they must train their bodies in order to be able to use the Quirk. This is because, unlike other people who are born with adaptations, they are born with none.
Currently, only three people in the entire franchise are confirmed to have been born totally Quirkless: Izuku Midoriya, Toshinori Yagi, and Knuckleduster. However, it is possible to steal a Quirk, as seen with All For One. This rendered Pro Hero Tomoko Shiretoko Quirkless, despite being born with a Quirk.