What is the meaning of ⟨_⟩?

Encloses the inner product of two vectors.

Denotes the expectation value of something.

Encloses orthographic representation.

Encloses restored writing that was mistakenly omitted by the original scribe.

Encloses titles of books when embedded within a book title.

Encloses titles of sections of a book.

Encloses titles of books.

Source: wiktionary.org