Kekkei Genkai (血継限界; Literally meaning "a technique limited to inheritance by blood" or "Bloodline Limit") is the name given to an ability passed down to a person genetically. Whilst it is possible for one person to have several of them, a specific ability is usually passed down (and kept within) specific clans, which then become known for their use of it. The ability itself can vary from jutsu that work through the eye - known as dōjutsu - to an advanced chakra nature such as Magnet Release.
The vast majority of kekkei genkai would be considered impossible without having the ability passed down to a person, though it is possible to gain them by transplants or DNA re-organisation. Both of these would require a person that already possesses the ability, and even then it might not be successful. Despite most kekkei genkai being specific to a certain clan, some abilities have been seen in various shinobi that have no apparent blood relation, suggesting that some of them can be caused by mutations in the body regardless of family. Likewise, some abilities may be considered specific to one clan but are found to be a rarity even within them.