Barrier

A is a membrane of either spiritual or demonic energy. Barriers can be cast over a wide area, as with Mount Hakurei, around an individual, or en masse. Barriers can protect the one who erected it from all sorts of dangers; such as physical intrusions or merely concealing one's presence. The strength of a barrier depends on the owner's spiritual or demonic power. Less powerful barriers can be broken into by throwing another barrier of the same or greater power against it. Barriers can also be broken through other powerful mean's of attack as well (e.g. sacred arrows).

Naraku
Naraku, over the course of the series, made use of three different types of barriers, allowing him to either conceal his presence or shield against physical onslaughts. Even some of his incarnations (i.e. the Infant, Hakudōshi and Byakuya) were able to erect barriers.

Kikyō
Kikyō can focus her spiritual power into extremely durable energy shields or screens. Under a barrier, she creates areas that neither humans nor demons can break into. When a human makes a barrier, even an ascetic monk must concentrate hard, but in Kikyō's case, she can maintain a barrier in her sleep.

Hakushin
Hakushin was a high-level Buddhist priest and living Buddha who created the barrier around Mount Hakurei. His barrier hindered even Sesshōmaru, a very powerful Daiyōkai, and was able to cancel out the demonic powers of the yōkai sword, Tōkijin, and the protective properties of the Robe of the Fire-Rat; even the poison Miroku takes in from the saimyōshō was purified, saving him from certain death. The barrier's purification abilities were so great that it forces Inuyasha into his human form as a result of the suppression of his yōkai blood.

Tessaiga's Sheath
The sheath of Tessaiga also has the ability to create barriers to protect the wielder from powerful attacks.

Blood Coral Crystal
The Blood Coral Crystal, is a red orb and a precious heirloom passed down to each guardian of the Hyakki Mori clan barrier, it gives them the ability to put up a powerful barrier and it stores all the power of each generation of guardians like Shiori's grandfather Taigokumaru and her father Tsukuyomaru.

Trivia

 * The colors of the barriers seem to be violet for evil characters, and blue for good characters. It doesn't seem to be based on demonic or spiritual energy, as Tessaiga's barrier is blue and Naraku, Hakudōshi, the Infant and Byakuya's are violet.