Short answer
Common examples of esoteric knowledge include Kabbalah and its Tree of Life, Hermetic correspondences, alchemy, astrology and tarot symbolism, initiatory ritual such as that of Freemasonry, number symbolism from the Pythagorean tradition, and Gnostic teachings about the soul’s origin. Each is “esoteric” because it is traditionally seen as inner, symbolic or reserved for people who are prepared to receive it.
What makes knowledge esoteric
Three features recur. The knowledge is treated as having an inner layer behind its public form. It is usually transmitted from teacher to student, sometimes in stages. And it is meant to change the learner, not just inform them. Scholars of Western esotericism use similar criteria. The word does not guarantee that the content is true, or that it is actually secret; some of these traditions are now widely published.
Examples from the Western tradition
Kabbalah and the Tree of Life
Kabbalah is the Jewish mystical tradition, taught for centuries to students prepared by study and observance. Its diagram of ten sephiroth was later adopted by Christian and occult writers; see the Hermetic Kabbalah.
Hermetic correspondence
The idea that the heavens, the earth and the human being mirror one another comes from Hermetic literature. Its best-known slogan, “as above, so below,” is explained in the Emerald Tablet article.
Alchemy
The symbolic language of alchemy, with its stages, metals and planets, is a standard example because its authors often wrote in deliberately obscure terms. See the alchemical tradition.
Astrology and tarot
Astrology rests on correspondences between the sky and earthly events, and the tarot is read as a symbolic system. The tarot’s esoteric meanings were developed mainly by French and British occultists in the eighteenth and nineteenth centuries. Neither is considered scientifically validated, and they are discussed here as part of the tradition, not as proven methods.
Initiatory ritual
Freemasonry and orders such as the Golden Dawn use graded initiation with symbolic ceremony. In Masonic usage, the “esoteric work” is the part of the ritual shared only with members; see Freemasonry and esoteric knowledge.
Number symbolism
Pythagorean ideas that numbers reveal the structure of the cosmos, also the root of sacred geometry, have influenced a long line of later thinkers. The numerology of Kabbalah and the numbers of Masonic lore are later examples.
Gnostic cosmology
Gnostic accounts of a hidden divine realm, a flawed world and a saving knowledge are among the oldest. See Gnosticism and esotericism.
Esoteric knowledge outside the West
Other religions have comparable teachings. Examples often cited include Sufism in Islam, Tantra in South Asia, and Daoist inner practices. They are generally studied under different headings, and comparing them to Western esotericism requires care, since each grows from its own history.
A note on “secret” knowledge today
Much once-restricted material is now freely published, and modern books often present it in simplified or reinvented form. Be cautious about claims that a teaching is thousands of years old or has been “hidden by elites.” When you meet such a claim, check what the earliest surviving source actually says.